Recent Blog Articles

COVID-19 vaccines

July 20, 2021

What you need to know before getting vaccinated

The FDA has authorized three COVID-19 vaccines. As more people get their shots, we will learn how long protection lasts. And we will gain answers to important questions, including whether vaccinations reduce the risk of infecting others. The vaccines offer hope that an end to this pandemic is in sight.

Visit our Coronavirus Resource Center for more information on coronavirus and COVID-19.

Which vaccines has the FDA authorized for COVID-19?

On December 11, 2020, the FDA granted emergency use authorization (EUA) to an mRNA COVID-19 vaccine developed by Pfizer and BioNTech. One week later, the FDA granted EUA to an mRNA COVID-19 vaccine developed by Moderna. The Johnson & Johnson adenovirus vaccine was granted EUA by the FDA in late February 2021. Use of the Johnson & Johnson vaccine resumed on April 23, 2021, after a temporary pause.

Which COVID-19 vaccines has the FDA authorized for children?

On May 10, 2021, the FDA expanded its emergency use authorization (EUA) for the Pfizer/BioNTech COVID-19 vaccine to include adolescents 12 to 15 years old. Previously, the Pfizer vaccine was authorized for use in children 16 years and older. For now, this is the only vaccine authorized in the U.S. for anyone under age 18.

Pfizer has been conducting age de-escalation studies, in which its vaccine is tested in groups of children of descending age. The EUA’s expanded authorization was based on results from a Phase 3 trial of children ages 12 to 15. The trial enrolled 2,260 adolescents; half received the Pfizer mRNA vaccine, the other received a saltwater placebo.

The immune response in the vaccinated adolescent group was even stronger than that in vaccinated 16- to 25-year-olds enrolled in an earlier study. In addition, a total of 16 symptomatic cases of COVID-19 were reported during the trial, all in the placebo group, which meant that the vaccine had been 100% effective in preventing COVID-19.

Vaccine-related side effects were mild and included pain at the injection site, tiredness, headache, chills, muscle pain, fever, and joint pain. Pfizer has also started testing the vaccine in children younger than 12 years.

What side effects can I expect from the COVID vaccine?

Minor side effects are common after COVID-19 vaccination. Almost everyone experiences arm pain at the injection site. Other symptoms can include low grade fever, body ache, chills, fatigue, and headache.

You can expect to feel better within 24 to 48 hours. Some people feel too unwell to go to work or perform their usually daily activities during this period. Contact your doctor if your symptoms have not improved by the third day.

Moving your sore arm around may help to relieve discomfort. If you have a fever, drink plenty of fluids. Over the counter pain relievers like ibuprofen or acetaminophen can also help with fever, pain, and other discomfort. However, it’s best to not take a pain reliever right before getting your shot, because there is a chance this could blunt your immune response.

The good news: These side effects are a sign that the vaccine is working and that your body is building an antibody response. The currently available vaccines require two shots, and side effects are more likely to occur after the second shot.

I have a severe allergy. Can I get the mRNA COVID-19 vaccine?

There have been rare cases of people having a severe allergic reaction after receiving the mRNA COVID-19 vaccine. As a result, everyone getting an mRNA vaccine in the US must be observed for at least 15 minutes after getting their shot, so they can receive immediate medical treatment if they experience a severe allergic reaction.

Despite the small risk, most people with a history of severe allergy can safely get the mRNA COVID-19 vaccine. This includes people who are allergic to food, pollen, bee stings, and medications taken by mouth. If you have a history of severe allergy, tell the person administering your vaccine. You will be observed for at least 30 minutes, instead of the usual 15 minutes.

If you have a history of allergic reactions to injectable medications or other vaccines, the CDC recommends asking your doctor if you should get one of the currently available mRNA vaccines.

There are some people who should not get an mRNA COVID vaccine. You should not get one if you are allergic to any components of the mRNA vaccine, which include polyethylene glycol (PEG) and polysorbate. And you should not get the second dose of an mRNA vaccine if you had an allergic reaction within the first 30 minutes after receiving the first vaccine dose.

If you have questions regarding the safety of the COVID vaccine for you, your best option is to talk to your doctor.

Are there any groups of people who should not get the mRNA COVID-19 vaccine?

There are only a few groups of people who should not get the mRNA vaccine. They include people who:

  • are allergic to any components of the mRNA vaccine, which include polyethylene glycol (PEG) and polysorbate
  • had an allergic reaction within the first 30 minutes after receiving the first dose of an mRNA COVID-19 vaccine.

If you have a history of allergic reactions to other (non-COVID) vaccines or to injectable medications, ask your doctor if it is safe for you to get one of the currently available mRNA vaccines.

People with weakened immune systems, including those with autoimmune conditions and those taking immune-suppressing treatment, are at increased risk for severe COVID illness. People in these groups can get an mRNA vaccine. However, there may be other safety considerations. If you fall into this category, talk to your doctor about your individual risks and benefits.

Finally, it is possible that convalescent plasma or monoclonal antibodies, both of which are used to treat COVID-19, could reduce the effectiveness of the vaccine. If you have received either of these treatments, wait 90 days before getting your vaccine.                                                                                             

How long will protection from the COVID-19 vaccines last?

Both the Pfizer/BioNTech and Moderna mRNA vaccines protect against COVID-19 for at least six months. This represents the minimum period of protection; only with time will we learn how much longer protection lasts. (The Johnson & Johnson vaccine has not been available long enough to collect six months of data.)

In a press release, Pfizer/BioNTech announced that their vaccine has been 91.3% effective in preventing COVID-19 up to six months after the second dose of the two-dose regimen. It was also found to be 95.3% to 100% effective at preventing severe disease (the number varied slightly depending on which definition of "severe disease" was used in the analysis).

Also in a press release, Moderna announced that its vaccine is more than 90% effective against all COVID-19 cases and more than 95% effective against severe cases of COVID-19 up to six months following the second dose of the two-dose regimen. In addition, a study published in NEJM found that antibodies created by the body in response to the Moderna vaccine continued to exist at high levels for the same period of time.

What are breakthrough infections, and why do they happen?

A breakthrough infection is one that occurs after full vaccination.

In an article published in MMWR, the CDC reported 10,262 breakthrough infections through the end of April 2021. By that point, more than 100 million Americans had received the COVID-19 vaccine.

The vast majority of breakthrough infections were asymptomatic, mild, or moderate. About 1,000 people with breakthrough infections were hospitalized, and 160 died, though the hospitalizations and deaths were not always related to COVID-19.

These numbers tell us that the vaccines are doing a good job preventing infection and severe illness. None of the vaccines were 100% effective in clinical trials, so a small number of breakthrough infections was expected.

What is the connection between the Johnson & Johnson COVID-19 vaccine and Guillain-Barré syndrome?

In July 2021, the FDA warned that the Johnson & Johnson (J&J) COVID-19 vaccine may lead to a small but increased risk of Guillain-Barré syndrome (GBS). GBS is a disorder in which the body destroys its own nerve cells. It can lead to muscle weakness and sometimes paralysis, trouble breathing, or, rarely, death. The risk of developing GBS after receiving the J&J vaccine is very low, and benefits of vaccination still far outweigh the risks.

The FDA’s warning was based on preliminary reports of about 100 cases of GBS that occurred in people who had received the J&J vaccine. To date, about 12.8 million people in the US have gotten the J&J vaccine. Most of the reported cases occurred in men ages 50 years and older, about two weeks after vaccination.

Almost all required hospitalization because GBS can affect the nerves to chest muscles and the diaphragm, making it difficult to breathe. Most people recover completely from GBS within a few months, but some never regain full strength.

Seek immediate medical attention if you experience any of the following symptoms after receiving the J&J COVID-19 vaccine:

  • weakness or tingling sensations, especially in the legs or arms, that’s worsening and spreading to other parts of the body
  • difficulty walking
  • difficulty with facial movement, including speaking, chewing, or swallowing
  • double vision or inability to move eyes
  • difficulty with bladder control or bowel function.

Why did the FDA and CDC recommend pausing use of the Johnson & Johnson COVID-19 vaccine?

On April 13, 2021, the FDA and CDC jointly recommended pausing use of the Johnson & Johnson COVID-19 vaccine until the agencies could review the cases of a rare but serious type of blood clot that were reported in six women who received the Johnson & Johnson vaccine. On April 23, the FDA and CDC recommended lifting the pause, and allowing use of the vaccine to resume. They made this decision after their medical and scientific teams examined the data and determined that the vaccine’s known and potential benefits outweigh its known and potential risks.

The Johnson & Johnson vaccine is once again available for use in the US, for anyone ages 18 and older. However, the vaccine’s label and factsheet will now warn of the rare risk of developing blood clots involving blood vessels in the brain, abdomen, and legs, along with low levels of blood platelets. (This combination is being referred to as thrombosis and thrombocytopenia syndrome, or TTS, an abbreviation of the medical terms for the two conditions.) The label and factsheet also list symptoms of TTS and urge anyone who experiences them after receiving the Johnson & Johnson vaccine to seek immediate medical attention.

Since the FDA granted emergency use authorization (EUA) for the Johnson & Johnson vaccine in late February, more than 8 million doses of the one-dose vaccine have been given. A total of 15 cases of TTS were reported through April 23rd; all occurred in women between the ages of 18 and 59, between six and 15 days after vaccination.

While the risk of experiencing a blood clot after vaccination is very low, anyone who experiences the following symptoms within three weeks of receiving the Johnson & Johnson vaccine should contact their doctor and seek immediate medical care:

  • severe headache
  • backache
  • blurred vision
  • fainting
  • seizures
  • severe pain in your abdomen or stomach
  • severe pain in your chest
  • leg swelling
  • shortness of breath
  • tiny red spots on the skin
  • new or easy bruising or bleeding.

The clotting issue reported with the Johnson & Johnson vaccine is similar to the rare cases of unusual blood clots that the European Medicines Agency linked to the AstraZeneca vaccine.

I recently got the Johnson & Johnson COVID-19 vaccine. Am I at risk for a blood clot?

More than 8 million doses of the Johnson & Johnson COVID-19 vaccine have been given in the US since it received emergency use authorization from the FDA in late February 2021. Through April 23rd, there have been 15 reported cases of a serious blood clotting issue. The FDA and CDC have called the risk of experiencing a blood clot after vaccination "very low."

The 15 reported cases all occurred in women between the ages of 18 and 59, between six and 15 days after vaccination. All experienced blood clots involving blood vessels in the brain, abdomen, and legs, along with low levels of blood platelets. (This combination is being referred to as thrombosis and thrombocytopenia syndrome, or TTS, an abbreviation of the medical terms for the two conditions.)

Contact your doctor and seek immediate medical care if you experience any of the following symptoms within three weeks of receiving the Johnson & Johnson vaccine:

  • severe headache
  • backache
  • blurred vision
  • fainting
  • seizures
  • severe pain in your abdomen or stomach
  • severe pain in your chest
  • leg swelling
  • shortness of breath
  • tiny red spots on the skin
  • new or easy bruising or bleeding.

How have the vaccines performed the real world? Are they working as well as they did in clinical trials?

In a study published online in MMWR, the CDC reported that people who had received both doses of either mRNA vaccine (Pfizer/BioNTech or Moderna) were 90% less likely to get infected with COVID-19 than people who were not vaccinated. In clinical trials, the vaccines were 95% (Pfizer/BioNTech) and 94.1% (Moderna) effective.

The MMWR study enrolled 3950 adults who were at high risk of exposure to the virus due to their jobs and followed them for 13 weeks, beginning in mid-December 2020. About 63% of the study participants received both doses of vaccine during the study period and an additional 12% received one dose during the study. Each week, the study participants sent a nasal swab for PCR testing, and they reported any symptoms, allowing the researchers to identify both symptomatic and asymptomatic infections.

A total of 180 COVID-19 infections occurred during the study period. Of these, 161 infections were in people who were unvaccinated, compared to three infections in people who were fully vaccinated. Sixteen people who had received only their first vaccine dose at least two weeks earlier were infected. While this translates to one dose preventing 80% of infections, the study did not measure how well the vaccine protects people who do not get the second dose.

The researchers did not comment on variants, and the study was not designed to measure the effectiveness of the vaccines against them. However, it’s worth noting that several viral variants were circulating during the study period.

Have the COVID vaccines effectively protected older adults in the real world?

A small study conducted by the CDC and published in MMWR reports that the two mRNA vaccines (Pfizer/BioNTech and Moderna) have been as effective in older adults in the real world as they were in the clinical trials that led to authorization of their use.

The MMWR study was based on 417 patients, ages 65 years and older, who were admitted to a hospital for COVID-like symptoms between early January and late March 2021. All study participants were tested for COVID-19 and asked about their COVID vaccination history.

Researchers compared those who tested positive for COVID-19 and those who had another illness, and also looked at each of the participants COVID vaccination history. Based on their comparisons and calculations, the researchers found that fully vaccinated people reduce their risk of COVID-related hospitalization by 94%. (Study participants were considered fully vaccinated two weeks after their second dose.) In clinical trials, the vaccines were 95% (Pfizer/BioNTech) and 94.1% (Moderna) effective. Effectiveness dropped to 64% in those who were partially vaccinated (defined as more than 14 days after the first dose, but less than two weeks after the second dose.)

Importantly, the vaccines did not protect against COVID-related hospitalization during the first two weeks after the first vaccine dose.

Why are some people hesitant about getting a COVID vaccine?

It’s natural to want to make an informed decision about the COVID-19 vaccine. Fortunately, the safety of the vaccine seen in the clinical trials has been verified by the real-world results.

Warp speed? Yes, the speed with which the COVID-19 vaccines were developed was incredibly fast. But the science and technology to make it happen came after decades of research.

COVID-19 mRNA vaccines are new, but scientists have been studying mRNA for many years. They cannot change your DNA. The mRNA sends a message to the body’s cells to make only a specific protein, not a virus, that stimulates the immune system to give you protection. Experts predict that most vaccines in the future will use mRNA technology.

If you still have doubts about the vaccine, we recommend this short video produced by the Black Women’s Health Imperative.

Some communities, particularly communities of color, may have historical reasons to doubt the health care system. But these communities have also been among the hardest hit by this pandemic, and like all of us, have a lot to gain from vaccination.

The Black Women’s Health Imperative is a national organization dedicated to improving the health and wellness of Black women and girls.

I’m vaccinated. What can I do now?

In May 2021, the CDC released guidance that fully vaccinated people can now participate in all indoor and outdoor activities without a mask and without physical distancing.

Just remember that you’ll still need to follow federal, state, tribal, and local laws, and workplace or business requirements around mask wearing and physical distancing.

For people who are not fully vaccinated, the CDC continues to recommend mask wearing and other preventive measures in some outdoors settings and in most indoor settings.

Will the COVID-19 vaccines work against the new coronavirus variants?

Mutations have led to several new, concerning coronavirus variants, including the B.1.1.7 (Alpha) variant, which was first detected in the United Kingdom; the B.1.351 (Beta) variant, which originated in South Africa; the P.1 (Gamma) variant, which originated in Brazil, and the B.1.617.2 (Delta) variant, which was first documented in India. Emerging evidence suggests that the current vaccines are effective against at least some of these variants.

Two peer-reviewed and published studies have found the Pfizer/BioNTech vaccine to be extremely effective against the Alpha and the Beta variants. The first study, published in NEJM, looked at data from more than 200,000 people from the country of Qatar between early February and late March 2021. During that time, the Alpha and Beta variants were responsible for nearly all COVID cases in that country. The researchers found that in people who were fully vaccinated, the Pfizer vaccine was 97.4% effective at preventing severe, critical, or fatal disease caused by the Alpha or Beta variants.

The second study, published in The Lancet, looked at more than 200,000 COVID infections in Israel from late January to early April 2021, when the Alpha variant accounted for more than 90% of infections. The researchers found that in fully vaccinated people, the Pfizer vaccine was nearly 97% or more effective at preventing symptomatic COVID-19, COVID-related hospitalization, severe or critical COVID-19, or COVID-related death.

Newer research has also found the Pfizer/BioNTech vaccine to be effective against the Delta variant, which is now the dominant variant in the US. A study from Public Health England found that full vaccination with the Pfizer/BioNTech vaccine was 88% effective at protecting against symptomatic illness caused by the Delta variant, and 96% effective at preventing hospitalization due to the Delta variant.

Studies on the effectiveness of the Moderna vaccine are ongoing. In the lab, it has performed well against the Alpha variant, compared to the original, unmutated virus. However, it was less effective against the Beta and Delta variants compared to the unmutated virus and the Alpha variant. More study is needed to see if the laboratory results hold up in real-world conditions.

It's important to remember that both the Pfizer/BioNTech and Moderna vaccines are extremely effective — 95% and 94.1%, respectively. Even with some possible decrease in effectiveness against variants, they will still provide excellent protection against severe illness.

Another study, published in Nature, looked at blood samples from people who received the Johnson & Johnson adenovirus vaccine. It found that the vaccine produced a less robust antibody response against the Beta and Gamma variants compared to the response to the original SARS-CoV-2 virus. However, the T cell immune response held up, particularly among the type of T cells that prevent the virus from spreading within the body. This immune response should protect against developing severe symptoms if a person does get infected. Johnson & Johnson also released data from a small, unpublished laboratory study, which found a strong neutralizing antibody response against the Delta variant in people who had received the single-shot J&J vaccine. The response was sustained for at least eight months, the duration of the study.

What do we know about the Moderna COVID-19 vaccine that the FDA has authorized for emergency use?

On December 18, 2020, the FDA granted emergency use authorization (EUA) to an mRNA COVID-19 vaccine developed by Moderna in collaboration with scientists from the NIH. The vaccine is approved for use in people 18 years and older. This is the second COVID-19 vaccine authorized for use in the US. (The Pfizer/BioNTech vaccine received EUA during the prior week.)

The FDA granted EUA based on their own analysis of efficacy and safety data, as well as on the recommendation made by the Vaccines and Related Biological Products Advisory Committee (VRBPAC). VRBPAC is a group of outside experts in infectious disease, vaccinology, microbiology, immunology, and other related fields.

In briefing documents submitted to the FDA, the Moderna vaccine showed an overall efficacy of 94.1% in preventing COVID-19. This study enrolled 30,400 adults; half received the vaccine, half received a saltwater placebo shot. There were 196 infections among the study participants. Of these, 185 were in the placebo group and 11 were in the vaccine group. All 30 cases of severe COVID occurred in the placebo group, strongly suggesting indicating that the vaccine reduces risk of severe illness.

The vaccine was similarly effective in people older and younger than 65, in men and women, in people with and without medical conditions that put them at high risk for severe illness, and in different racial and ethnic groups.

The most common vaccine side effects were pain at the injection site, fatigue, headache, muscle pain, joint pain, and chills.

This vaccine requires two doses, spaced four weeks apart. In January 2021, the CDC released guidance stating that while the second dose should be administered as close to the recommended four-week interval as possible, it may be scheduled for administration up to six weeks (42 days) after the first dose. We do not yet know how long immunity from this vaccine will last.

The Moderna vaccine contains synthetic messenger RNA (mRNA), genetic material that enters human cells and instructs them to produce the spike protein found on the surface of the SARS-CoV-2 virus. The body recognizes the spike protein as an invader and produces antibodies against it. Soon after, the cell breaks down the mRNA into harmless pieces. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

Unlike the Pfizer vaccine, which requires ultracold storage, the Moderna vaccine can be stored at normal freezer temperatures. This may allow for wider distribution to pharmacies and other facilities that are not equipped for ultracold storage.

What do we know about the Pfizer/BioNTech COVID-19 vaccine that the FDA has authorized for emergency use?

In May 2021, the FDA expanded its emergency use authorization (EUA) for the Pfizer/BioNTech COVID-19 vaccine to include adolescents 12 to 15 years old. Previously, the Pfizer vaccine was authorized for use in children 16 years and older

The FDA granted its original EUA based on their own analysis of efficacy and safety data as well as on the recommendation made by the Vaccines and Related Biological Products Advisory Committee (VRBPAC) on December 10th. VRBPAC is a group of outside experts in infectious disease, vaccinology, microbiology, immunology, and other related fields.

Results from the Pfizer/BioNTech vaccine trial were published in the New England Journal of Medicine. The data showed that the vaccine reduced the risk of COVID-19 by 95%. The trial enrolled nearly 44,000 adults, each of whom got two shots, spaced three weeks apart; half received the vaccine and half got a placebo (a shot of saltwater). Of the 170 cases of COVID-19 that developed in the study participants, 162 were in the placebo group and eight were in the vaccine group. Nine of the 10 severe COVID cases occurred in the placebo group, suggesting that the vaccine reduced risk of both mild and severe COVID.

According to the NEJM article, the vaccine was similarly effective in study participants of different races and ethnicities, body weight categories, presence or absence of coexisting medical conditions, and ages (younger and older than 65). It's worth noting that the FDA felt comfortable authorizing the vaccine for 16- and 17-year-olds, even though the number of teens enrolled in the study was small.

None of the study participants experienced serious side effects. However, most did have pain at the injection site. Also, about half of those receiving the vaccine reported mild to moderate fatigue or headache or both. Chills and fever were also fairly common. Symptoms almost always resolved within 24 to 48 hours.

The expanded EUA was based on results from a Phase 3 trial testing the Pfizer vaccine in children ages 12 to 15. The trial enrolled 2,260 adolescents; half received the Pfizer mRNA vaccine, the other received a saltwater placebo. The immune response in the vaccinated adolescent group was even stronger than that in vaccinated 16- to 25-year-olds enrolled in an earlier study. In addition, a total of 16 symptomatic cases of COVID-19 were reported during the trial, all in the placebo group. Vaccine-related side effects were mild and included pain at the injection site, tiredness, headache, chills, muscle pain, fever, and joint pain.

This vaccine requires two doses, spaced three weeks apart. In January 2021, the CDC released guidance stating that while the second dose should be administered as close to the recommended three-week interval as possible, it may be scheduled for administration up to six weeks (42 days) after the first dose. Although the vaccine appears to provide reasonable protection after the first dose, it provides stronger protection after two doses. We do not yet know how long immunity from this vaccine will last.

The Pfizer/BioNTech vaccine is an mRNA vaccine. The vaccine contains synthetic messenger RNA (mRNA), genetic material that contains instructions for making proteins. Inside the body, the mRNA enters human cells and instructs them to produce a single component of the SARS-CoV-2 virus — the "spike" protein found on the virus's surface. The body recognizes the spike protein as an invader and produces antibodies against it. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness. mRNA vaccines must be stored at very cold temperatures; improperly stored vaccines can become inactive.

Healthcare workers and residents and staff of long-term care facilities will be the first to get the vaccine.

What do we know about Johnson & Johnson's adenovirus vaccine?

On February 27, 2021, the FDA granted emergency use authorization (EUA) for Johnson & Johnson's single-shot adenovirus vaccine, following the recommendation from the FDA's Vaccines and Related Biological Products Advisory Committee (VRBPAC). The vaccine is authorized for use in people 18 years and older.

In April 2021, the FDA and CDC jointly recommended pausing use of the Johnson & Johnson COVID-19 vaccine until the agencies could investigate the cases of an extremely rare but serious type of blood clot that were reported in six women who received the Johnson & Johnson vaccine. After examining the data and determining that the vaccine’s known and potential benefits outweigh its known and potential risks, the FDA and CDC recommended lifting the pause, and allowing use of the vaccine to resume.

The Johnson & Johnson vaccine was once again made available for use in the US, for anyone ages 18 and older. However, the vaccine’s label and factsheet will now warn of the rare risk of developing blood clots involving blood vessels in the brain, abdomen, and legs, along with low levels of blood platelets. (This combination is being referred to as thrombosis and thrombocytopenia syndrome, or TTS, an abbreviation of the medical terms for the two conditions.) The label and factsheet also list symptoms of TTS and urge anyone who experiences them after receiving the Johnson & Johnson vaccine to seek immediate medical attention.

In July 2021 the FDA added another warning to the vaccine’s factsheet regarding a very small but increased risk of Guillain-Barré syndrome (GBS) following vaccination. GBS is a disorder in which the body destroys its own nerve cells. It can lead to muscle weakness and sometimes paralysis, trouble breathing, or, rarely, death. The revised factsheet lists symptoms of GBS and urges anyone who experiences them after vaccination to seek immediate medical attention. Once again, the benefits of vaccination outweigh the small increased risk.

The EUA was based on clinical trial data that found the vaccine to be 85% effective at preventing severe disease, including prevention of hospitalization and death due to COVID-19. The vaccine also appeared to be more than 80% effective at preventing severe COVID disease in South Africa, where the B.1.351 variant is more common. Early data suggest the vaccine may also result in milder illness with fewer symptoms in those who develop COVID-19 after vaccination.

Johnson & Johnson's ongoing phase 3 randomized, double-blind, placebo-controlled clinical trial, is designed to evaluate the safety and effectiveness of the vaccine compared to placebo in adults ages 18 and older. The study enrolled nearly 45,000 participants; half received the COVID-19 vaccine and the other half received a placebo.

Protection was generally consistent across race and age groups. Worldwide, the study participants were 59% White/Caucasian, 45% Hispanic and/or Latinx, 19% Black/African American, 9% Native American, and 3% Asian. In addition, 34% of participants enrolled in the study were over age 60, and 41% had at least one medical condition that put them at increased risk for severe COVID illness. Results suggest the vaccine may be less effective in people 60 years and older with comorbid conditions like diabetes or heart disease; however, no COVID-related deaths or medical interventions occurred in this population of vaccinated study participants.

Common vaccine side effects included mild to moderate pain at the site of injection, headache, fatigue, and muscle aches. A few rare, unexpected side effects also occurred: 15 vaccinated study participants and 10 volunteers who got a placebo developed conditions related to blood clots; six vaccinated volunteers (but none in the placebo group) developed a ringing in the ears called tinnitus; and there were eight cases of hives among vaccine recipients, compared to three among those who received placebo.

The Johnson & Johnson vaccine uses a modified, harmless form of a human common-cold adenovirus to deliver into the body the genetic instructions for making the coronavirus spike protein. Human cells use the instructions to make copies of the spike protein. The body recognizes the spike protein as an invader and produces antibodies against it. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

In addition to the advantage of having a one-dose vaccine, the other point in favor of the J&J vaccine is ease of storage. It can be safely refrigerated for months and frozen for even longer, unlike the mRNA vaccines, which must be frozen (one at ultracold temperatures).

What do we know about the Novavax COVID-19 vaccine?

The Novavax COVID-19 vaccine appears to be effective and safe, according to an analysis of phase 3 trial results released by the company in a press release. The trial found the vaccine to be 90% effective overall, and 100% effective against moderate and severe disease. The vaccine requires two doses, given three weeks apart, and may be stored using standard refrigeration.

The PREVENT-19 trial enrolled 29,960 participants, ages 18 years and older, across the US and Mexico. Two-thirds of the participants received the Novavax vaccine and one-third received a placebo. The trial was randomized, and observer-blinded, meaning the study participants and those evaluating the study endpoints did not know which participants received the vaccine.

Between January 25 and April 30, 2021, there were 77 confirmed cases of COVID-19 among the study participants. Of these, 63 occurred in the placebo group and 14 occurred in the vaccine group. All of the cases that occurred in the vaccine group were mild. Of those that occurred in the placebo group, 10 were moderate and four were severe. This translated to 100% efficacy against moderate and severe disease. The vaccine was 91% effective in people at high risk, which the study defined as people 65 years or older, with a medical condition that increased risk of severe COVID illness, or whose jobs increased their risk of exposure to COVID-19.

Side effects from the vaccine included arm pain at the injection site, fatigue, headache, and muscle pain.

The Novavax vaccine is a recombinant protein vaccine. Similar to COVID vaccines that are already available in the US, it stimulates the body to produce antibodies against the spike protein found on the outer surface of the SARS-CoV-2 virus. The Novavax vaccine contains spike protein produced in a laboratory along with an adjuvant to boost the human immune response.

The vaccine is already being tested in adolescents ages 12 to 17.

What do we know about the AstraZeneca adenovirus COVID-19 vaccine?

An analysis of Phase 3 clinical trial data suggests that the COVID-19 vaccine developed by AstraZeneca and the University of Oxford is effective and safe. AstraZeneca presented interim results in a press release on March 22, 2021. Within a day, however, the National Institutes of Allergy and Infectious Diseases (NIAID) released a statement expressing concern that AstraZeneca's analysis may contain outdated information and may provide "an incomplete view of the efficacy data." On March 25th, AstraZeneca released a new analysis, also in a press release, that included more recent trial data. The FDA will have the opportunity to carefully examine and conduct its own analysis of all of the trial data when AstraZeneca applies for emergency use authorization (EUA), which is expected to happen in the coming weeks.

According to AstraZeneca's most recent analysis, vaccine recipients were 76% less likely to develop symptomatic COVID infection than those who received a placebo. The vaccine was also 100% effective for protecting against severe COVID and hospitalization; there were eight cases of severe COVID in the placebo group, compared to none in the vaccine group.

The analysis looked at results from a trial of about 32,000 participants from the US, France, Chile, and Peru. Two-thirds of the study participants were randomly selected to received two doses of the vaccine, spaced four weeks apart. The remaining one-third of participants got a saltwater placebo.

The trial did not raise any safety concerns. However, the European Medicines Agency (EMA) conducted a specific review of blood clots — a cause of concern in Europe, where the vaccine is already available. On April 7, 2021 the EMA concluded that there is a link between the AstraZeneca vaccine and very rare cases of unusual blood clots. However, the EMA went on to say that the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects. Most cases reviewed by the EMA occurred within two weeks of vaccination in women younger than 60 years.

The vaccine was 85% effective in people ages 65 and older. It was also similarly effective across people of different ethnicities.

One important question is how effective this vaccine is against SARS-CoV-2 variants. A NEJM study reporting results from the South Africa arm of the AstraZeneca trial concluded that the AstraZeneca vaccine did not protect against mild to moderate COVID-19 caused by the B.1.351 (South Africa) variant.

The AstraZeneca vaccine uses a modified, harmless form of a chimpanzee common-cold adenovirus to deliver the genetic code for the coronavirus spike protein into the body. Human cells read the code and make the spike protein, prompting the immune system to produce antibodies against it, and preparing the body for attack if it later encounters SARS-CoV-2.

Johnson & Johnson's COVID-19 vaccine, which received EUA in February 2021, is also an adenovirus vaccine. Both the AstraZeneca and Johnson & Johnson adenovirus vaccines can be safely refrigerated for several months, unlike the mRNA vaccines, which must be frozen.

Should I get the COVID-19 vaccine if I already had COVID-19?

New research suggests that people who have already gotten sick with COVID-19 benefit from the vaccine, possibly even more than vaccinated people who were not previously infected.

A study published in NEJM found that people who were previously infected with SARS-CoV-2 and then received a single dose of either mRNA vaccine (Pfizer/BioNTech or Moderna) experienced a rapid immune response and generated at least as many, or more, antibodies as previously uninfected people who had received two doses of vaccine.

And a study published in Science compared the effects of the Pfizer vaccine in people with and without prior infection. The researchers found that previously infected people who were vaccinated had a stronger immune response against the B.1.1.7 and B.1.351 variants than previously uninfected people who were vaccinated.

Will the COVID vaccine prevent me from infecting others?

Increasing evidence suggests that a person who has been vaccinated is less likely to infect others.

Two studies released in February pointed in this direction. The first study looked at viral load — the amount of SARS-CoV-2 virus in a person's nose and throat, which can be spread to others. The study found that the higher a person's viral load, the more likely they were to spread the infection; the lower a person's viral load, the less likely they were to spread the infection. Results from the second study suggested that people who became infected with SARS-CoV-2 after getting a COVID vaccine had a lower viral load than people who were infected but had not had a COVID vaccine. Taken together, the studies suggest that the COVID vaccine protects against both infection and transmission.

A study published in March 2021 showed that people who received an mRNA vaccine had significantly less risk of asymptomatic infection than people who were unvaccinated. This is important because people who are infected but never go on to develop symptoms are responsible for an estimated 24% of SARS-CoV-2 transmission.

According to guidance from the CDC, fully vaccinated people can now participate in all indoor and outdoor activities without a mask and without physical distancing. If you are not fully vaccinated, continue to follow CDC guidance on masking and other prevention measures, to protect yourself and others.

Once I get the COVID-19 vaccine, can I stop taking other precautions?

According to new guidance from the CDC, fully vaccinated people can now participate in all indoor and outdoor activities without a mask and without physical distancing. For people who are not fully vaccinated, the CDC continues to recommend mask wearing and other preventive measures in some outdoors settings and in most indoor settings.

Whether you’re vaccinated or not, you’ll need to follow federal, state, tribal, and local laws, and workplace or business requirements around mask wearing and physical distancing.

The CDC has created a helpful chart illustrating the protection vaccination provides in all outdoor and indoor activities, and what prevention measures you still need to take if you are not vaccinated. To view the chart, click here.

What needed to happen before the FDA authorized a vaccine for COVID-19?

A successful COVID-19 vaccine had to safely and effectively stimulate the body's immune system to create antibodies that protect against the COVID-19 coronavirus. On December 11, 2020, the FDA granted emergency use authorization (EUA) to an mRNA COVID-19 vaccine developed by Pfizer and BioNTech. One week later, the FDA granted EUA to another mRNA COVID-19 vaccine, this one developed by Moderna. On February 27, 2021, the FDA granted EUA to an adenovirus vaccine developed by Johnson & Johnson. A number of other, possible COVID-19 vaccines are in various stages of development and testing.

Approval of a vaccine requires completion of the following steps:

  • Pre-clinical testing: Animals are infected with the virus. Scientists study their immune response to see what aspects of the immune response might be critical for protection. Normally, a vaccine is first tested in animals. However, in the setting of a pandemic such as this one, the animal testing stage can be skipped.
  • Phase 1 trials: A vaccine is tested in small groups of people to determine what dose safely and consistently stimulates the immune system. At this stage, scientists don't yet know if the immune response triggered by the vaccine will protect against the virus.
  • Phase 2 trials: The vaccine is given to hundreds or thousands of people. Scientists continue to focus on whether the vaccine is safe and produces a consistent immune response.
  • Phase 3 trials: These trials typically enroll tens of thousands of people. This is the first phase that involves a placebo group. It compares the number of people who get sick in the vaccine group to the number of people who get sick in the placebo group. This is the only phase that can show whether or not the immune response triggered by the vaccine actually protects against infection in the real world.

What are mRNA vaccines and how do they work to help prevent COVID-19?

mRNA, or messenger RNA, is genetic material that contains instructions for making proteins. mRNA vaccines for COVID-19 contain synthetic mRNA. Inside the body, the mRNA enters human cells and instructs them to produce the "spike" protein found on the surface of SARS-CoV-2, the virus that causes COVID-19. The body recognizes the spike protein as an invader, and starts producing antibodies against it. Soon after, the cell breaks down the mRNA into harmless pieces If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

Two mRNA vaccines, one created by Pfizer and BioNTech and another developed by Moderna, were granted emergency use authorization (EUA) by the FDA in December 2020.

Could an mRNA vaccine change my DNA?

An mRNA vaccine — the first COVID-19 vaccine to be granted emergency use authorization (EUA) by the FDA — cannot change your DNA.

mRNA, or messenger RNA, is genetic material that contains instructions for making proteins. mRNA vaccines for COVID-19 contain man-made mRNA. Inside the body, the mRNA enters human cells and instructs them to produce the "spike" protein found on the surface of the COVID-19 virus. Soon after a cell makes the spike protein, the cell breaks down the mRNA into harmless pieces. At no point does the mRNA enter the cell's nucleus, which is where our genetic material (DNA) lives.

The immune system recognizes the spike protein as an invader and produces antibodies against it. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

I am pregnant and plan to eventually breastfeed my baby. Is it safe for me to get a mRNA COVID-19 vaccine?

The CDC, American College of Obstetrics and Gynecology (ACOG), and Society for Maternal-Fetal Medicine agree that the new mRNA COVID-19 vaccines (Pfizer/BioNTech and Moderna) should be offered to pregnant and breastfeeding individuals. However, the World Health Organization (WHO) recommends against use of either mRNA vaccine in pregnant women, unless the benefit to an individual outweighs the potential vaccine risks. Experts, including the WHO, believe it is most likely safe to get an mRNA COVID-19 vaccine if you're breastfeeding. As always, your own doctor is in the best position to advise you based on your personal health risks and preferences.

Here are some factors to consider. First, although the actual risk of severe COVID-19 illness and death among pregnant individuals is very low, it is higher when compared to nonpregnant individuals from the same age group. In addition, research suggests that having COVID-19 might increase risk for premature birth, particularly for those with severe illness. Transmission of the virus from mother to baby during pregnancy is possible, but it appears to be a rare event.

Early mRNA vaccine trials did not deliberately include pregnant individuals. However, subsequent trials have enrolled women who are pregnant. One such study, conducted by the CDC, published reassuring preliminary results in NEJM. The trial looked at 3958 women who received an mRNA COVID-19 vaccine between mid-December 2020 and late February 2021. All of the women were pregnant when they received the vaccine or became pregnant after getting the vaccine, and enrolled in the v-safe pregnancy registry, which tracks pregnancy and infant outcomes. Most of the women included in this analysis got the vaccine during their second or third trimester. Of these women, 827 had completed their pregnancy by the end of the study period. There were 712 live births, and the number of miscarriages and other adverse outcomes, including preterm birth and low birth weight, were similar to pregnancy outcomes reported in studies before the COVID-19 pandemic.

Another small study, which compared vaccine efficacy in pregnant and non-pregnant women, was published in the American Journal of Obstetrics and Gynecology. The study found that mRNA vaccines effectively produce antibodies that protect against SARS-CoV-2 in women who are pregnant or breastfeeding, and that this immunity is passed from mother to newborn through the placenta and breast milk.

mRNA vaccines do not contain any virus, so they cannot cause COVID-19 in a woman or her baby. And our bodies quickly break down and eliminate mRNA particles used in the vaccine, so they are unlikely to reach or cross the placenta.

Who will get the first COVID-19 vaccines and who is next?

In mid-December, healthcare workers and residents and staff of long-term care facilities became the first group in the US to start getting the COVID-19 vaccine. The CDC has recommended that adults age 75 years and older and frontline essential workers be vaccinated next. They will be followed by adults aged 65 to 74, people aged 16 to 64 with medical conditions that put them at high risk for severe COVID-19, and remaining essential workers.

State and local authorities are not obligated to follow the CDC's recommendations; they may reprioritize the order in which they distribute the vaccines at their discretion.

There are about 21 million healthcare workers in the US, doing a variety of jobs in hospitals and outpatient clinics, pharmacies, emergency medical services, and public health. Another three million people reside or work in long-term care facilities, which include nursing homes, assisted-living facilities, and residential care facilities. COVID-19 has taken a heavy toll on residents of long-term care facilities. Frontline essential workers are those who cannot work from home and who may not be able to physically distance while working. They include first responders such as firefighters and police, teachers and day care workers, postal workers, and people who work in grocery stores.

The CDC's guidance is based on a recommendation from the Advisory Committee on Immunization Practices (ACIP), made up of experts in vaccinology, immunology, virology, public health, and other related fields. Their work is not limited to the COVID-19 vaccine; they broadly advise the CDC on vaccinations and immunization schedules.

When will children be able to get the COVID-19 vaccine?

In May 2021, the FDA expanded its emergency use authorization (EUA) for the Pfizer/BioNTech COVID-19 vaccine to include adolescents 12 to 15 years old. Previously, the Pfizer vaccine was authorized for use in children 16 years and older, while the Moderna and Johnson & Johnson vaccines are authorized for people 18 years and older. Moderna is also conducting age de-escalation studies, in which the vaccines are tested in groups of children of descending age. Johnson & Johnson plans to do the same.

The EUA’s extended authorization was based on results from a Phase 3 trial testing its vaccine in children ages 12 to 15. The trial enrolled 2,260 adolescents; half received the Pfizer mRNA vaccine, the other received a saltwater placebo. The immune response in the vaccinated adolescent group was even stronger than that in vaccinated 16- to 25-year-olds enrolled in an earlier study. In addition, a total of 16 symptomatic cases of COVID-19 were reported during the trial, all in the placebo group. Vaccine-related side effects were mild and included pain at the injection site, tiredness, headache, chills, muscle pain, fever, and joint pain. Pfizer has also started testing the vaccine in children younger than 12 years.

In June 2021, Moderna applied to the FDA for EUA of their mRNA vaccine for use in children ages 12 to under 18 years. The company announced promising results of a Phase 2/3 study in adolescents in May. The study enrolled 3,732 children ages 12 to 17. Two-thirds received two doses of the Moderna mRNA vaccine and one-third received a placebo. The immune response generated by the vaccine in adolescents was found to be at least as good as the immune response the vaccine generated in adults. Starting two weeks after the second vaccine dose, no cases of COVID-19 occurred in the vaccine group, compared to four cases in the placebo group. Vaccine side effects were mild to moderate, with injection site pain, headache, fatigue, muscle pain, and chills being the most common. The study did not identify any significant safety concerns. The results were announced in a press release. Moderna is also studying the vaccine in children between the ages of 6 months and 12 years.

The age de-escalation studies are done to confirm that the vaccines are safe and effective for each age group. They also identify the optimal dose, which must be effective, but with tolerable side effects.

The age de-escalation studies are smaller than the adult trials; rather than recruiting tens of thousands of participants, they will recruit 2,000 to 3,000 participants within each age group. As in the adult trials, some children in each trial will get a placebo.

The FDA will review data from the de-escalation trials to decide whether to authorize the vaccines for each age group.

I have been hearing about heart problems in kids and young adults following the COVID vaccine. Should I still get my child vaccinated?

There has a been a higher-than-expected number of heart inflammation cases after vaccination with the mRNA COVID-19 vaccines, particularly among boys and young men. However, the CDC still strongly recommends that all children 12 years and older be vaccinated.

As of July 12, 2021, 1,047 reports of myocarditis (inflammation of the heart muscle) and pericarditis (inflammation of the outer lining of the heart) had been reported in people under age 30, particularly in male teens and young adults, after vaccination with the Pfizer/BioNTech or Moderna mRNA vaccines The CDC’s Advisory Committee on Immunization Practices (ACIP) has said available data "suggest likely association of myocarditis with mRNA vaccination in adolescents and young adults." Even with the increased risk, heart inflammation is a rare occurrence.

Myocarditis and pericarditis after vaccination was most common in males ages 16 to 24. Cases tended to occur within several days after the second mRNA vaccine dose. Most people who developed myocarditis or pericarditis had mild cases and recovered completely after treatment.

If your child develops any of the following symptoms within a week of vaccination, seek medical care:

  • chest pain
  • shortness of breath
  • feeling like your heart is beating fast, fluttering, or pounding.

This or any other potential significant side effect of a vaccine is always of special concern in children. While the risk of COVID causing severe illness and hospitalization is much lower compared to adults, it’s important to remember that at least 491 children ages 0 to 17 years have died from COVID. In addition, long-term adverse health effects even after mild infection in children are now being recognized.

It’s understandable that many parents will be uncomfortable with either choice, vaccinate now or wait. But the rise of the more contagious and possibly more dangerous Delta variant, combined with the protection offered by herd immunity, supports the CDC recommendation that all children ages 12 year and older get vaccinated.

Who is eligible to get the COVID-19 vaccine, and who is next?

The CDC issued guidance for vaccine rollout based on recommendations from the Advisory Committee on Immunization Practices (ACIP).

Healthcare workers and residents and staff of long-term care facilities became the first group in the US to start getting the COVID-19 vaccine in mid-December 2020. The CDC recommended that adults ages 75 and older and frontline essential workers be vaccinated next, to be followed by adults ages 65 to 74, people ages 16 to 64 with medical conditions that put them at high risk for severe COVID-19, and remaining essential workers. Eventually, anyone who has not been vaccinated will be eligible for a COVID vaccine.

States across the US are at different stages of their vaccine rollouts.

What are the promising vaccines for COVID-19 under investigation?

Around the world, there are currently over 70 different COVID-19 vaccines in various stages of testing and development: phase 1 (safety), phase 2 (optimal dose, schedule, and proof of concept), and phase 3 (effectiveness, side effects) trials in humans.

In December 2020, the FDA granted emergency use authorization (EUA) to two COVID-19 vaccines, one developed by Pfizer/BioNTech and another developed by Moderna. These vaccines use a type of molecule called messenger RNA (mRNA) that can be mass-manufactured very rapidly. In these vaccines, mRNA induces human cells to make a protein that looks just like the spike protein that studs the surface of the coronavirus and enables it to enter human cells. The body recognizes the spike protein as an invader, and produces antibodies against it. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

In February 2021, the FDA granted EUA to Johnson & Johnson's adenovirus vaccine. However, on April 13, 2021, the FDA and CDC jointly recommended pausing use of the Johnson & Johnson COVID-19 vaccine until the agencies could investigate the cases of an extremely rare but serious type of blood clot that were reported in six women who received the Johnson & Johnson vaccine. On April 23rd, the FDA and CDC recommended lifting the pause, and allowing use of the vaccine to resume. They made this decision after their medical and scientific teams examined the data and determined that the vaccine’s known and potential benefits outweigh its known and potential risks.

The Johnson & Johnson vaccine uses a modified, harmless form of a human common-cold adenovirus to deliver into the body the genetic instructions for making the coronavirus spike protein. Human cells use the instructions to make copies of the spike protein. The body recognizes the spike protein as an invader and produces antibodies against it.

Two other vaccine approaches are also showing promise.

  • A vaccine created by AstraZeneca and the University of Oxford uses a modified, harmless form of a chimpanzee common-cold adenovirus to deliver the genetic code for the coronavirus spike protein into the body. Human cells read the code and make spike protein, which prompts the immune system to produce antibodies against it, preparing the body to attack the SARS-CoV-2 virus if it later infects the body. The company released promising interim phase 3 trial results in a press release in March 2021. However, on April 7, 2021, the European Medicines Agency announced a link between the AstraZeneca vaccine and very rare cases of unusual blood clots, similar to the clotting issue reported with the Johnson & Johnson vaccine. The EMA went on to say that the overall benefits of the vaccine in preventing COVID-19 outweigh the risks of side effects.
  • Another vaccine, created by Novavax, contains manufactured spike protein and an adjuvant; together, they stimulate an immune response. In a press release, Novavax released an analysis of phase 3 clinical trial results from the US and Mexico. The trial found the vaccine to be 90% effective overall, 100% effective against moderate and severe disease, and 91% effective in people at high risk. This vaccine requires two doses, given three weeks apart.

A central question for COVID-19 vaccines will be how long protection might last.

Blog posts

Visit our Coronavirus Resource Center for more information on coronavirus and COVID-19.

Image: Mongkolchon Akesin/Getty Images

Disclaimer:

As a service to our readers, Harvard Health Publishing provides access to our library of archived content. Please note the date of last review or update on all articles. No content on this site, regardless of date, should ever be used as a substitute for direct medical advice from your doctor or other qualified clinician.