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Coronavirus Resource Center

October 22, 2021

As coronavirus continues to spread, many questions and answers

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Below, you'll find answers to common questions all of us are asking. We will be adding new questions and updating answers as reliable information becomes available. Also see our blog posts featuring experts discussing coronavirus and COVID-19 and our glossary for relevant terms.

New questions and answers

Which COVID-19 vaccine booster should I get?

In October 2021, the FDA authorized, and the CDC approved, a single-dose booster for eligible recipients of all three COVID-19 vaccines (Pfizer/BioNTech, Moderna, and Johnson & Johnson). Individuals may select any vaccine for their booster, either the same as (homologous) or different than (heterologous) their initial vaccine(s).

The FDA’s authorization was based, in part, on an NIH study that has not yet been peer reviewed or published. The study enrolled 458 adults who had received one of the three available COVID-19 vaccines at least three months earlier. The study participants each got a booster dose: one-third received a Pfizer/BioNTech booster, one-third received a Moderna booster, and one-third received a Johnson & Johnson booster. The researchers looked at immune response, side effects, and safety of the nine resulting vaccine combinations.

The researchers found that any vaccine-plus-booster combination substantially increased the level of neutralizing antibodies (antibodies that block the COVID virus from infecting cells). However, the increase was greatest in those who followed a Johnson & Johnson vaccine with an mRNA vaccine, in particular the Moderna vaccine. The increase in neutralizing antibodies was fourfold for Johnson & Johnson followed by a Johnson & Johnson booster; 35-fold for Johnson & Johnson followed by a Pfizer/BioNTech booster, and 76-fold for Johnson & Johnson followed by a Moderna booster. However, the Moderna booster dose used in this study contains twice as much antigen as the dose that the FDA has authorized for the Moderna booster. And, importantly, the study was not designed to directly compare vaccine regimens.

The study uncovered no new safety concerns, and side effects were similar to those people have experienced with COVID vaccinations across all primary and booster vaccines.

Given this and other evidence, it makes sense for most people who started with the Johnson & Johnson vaccine to consider boosting with an mRNA vaccine (Moderna or Pfizer/BioNTech). On the other hand, if you started with the Moderna vaccine, you might think twice before mixing and matching.

The FDA’s mix-and-match authorization makes booster shots less dependent on which vaccines are available and gives people more choices. For example, if you had an unpleasant reaction to your initial vaccine, you could choose a different one for your booster. Or, if you don’t remember which vaccine you started with, you can still get a booster.

The FDA and CDC did not recommend that particular groups of people seek out or avoid specific vaccines. However, it may be reasonable for young women to ask their doctor whether they should avoid a booster dose of the Johnson & Johnson vaccine, which is associated with increased risk of a rare but serious type of blood clot that has been more common in women ages 18 to 49 years. In addition, there has a been a higher-than-expected number of heart inflammation cases after vaccination with the mRNA COVID-19 vaccines in male adolescents and young men. Those who fall into that category may want to ask their doctor if they should avoid a Pfizer/BioNTech or Moderna booster.

Anyone who initially received a Johnson & Johnson vaccine is eligible for a booster dose at least two months after their initial vaccine. Certain groups of people who originally received the Pfizer/BioNTech or Moderna vaccines are eligible for a booster after at least six months, depending on their age, health conditions, the type of work they do, and the type of place they live.

Who can get a COVID-19 vaccine booster shot?

In October 2021, the FDA authorized, and the CDC recommended, booster shots for eligible recipients of all three COVID-19 vaccines (Pfizer/BioNTech, Moderna, and Johnson & Johnson). Individuals may select any vaccine for their booster, either the same (homologous) or different (heterologous) than their initial vaccine(s).

Following is a description of who is eligible for COVID-19 vaccine boosters, based on which vaccine an individual initially received.

Pfizer/BioNTech or Moderna

If you have gotten two doses of the Pfizer/BioNTech or Moderna COVID-19 vaccine, you are eligible for a booster dose at least six months after completion of your initial vaccine series if you

Johnson & Johnson

Anyone age 18 years and older who received an initial dose of the J&J vaccine is eligible for a booster dose at least two months after the initial vaccine.

Can I get my flu shot at the same time as my COVID-19 booster shot?

According to the CDC, you can get the COVID-19 vaccine and another vaccine, including the flu shot, at the same time. The CDC’s guidance applies to the initial or booster dose of the COVID-19 vaccine.

Getting both vaccines at the same visit does not compromise the effectiveness of either immunization. Side effects are generally the same for vaccines given together or separately. Of course, if you get the shots in different arms, you could experience soreness in both arms.

Many clinics are now offering both vaccines at the same visit. It’s best to get your flu shot by the end of October.

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

Pfizer/BioNTech has announced that its COVID-19 vaccine is safe and effective in children ages 5 to 11 years. The company has submitted its data to the FDA and has applied for emergency use authorization (EUA) for this age group. An FDA advisory committee will meet to discuss the matter on October 26th.

In the Pfizer study, 2,268 children, ages 5 to 11, received two doses of the COVID-19 vaccine, spaced three weeks apart, while a smaller group of children received a placebo. The children in this study were given 10 microgram doses of the vaccine, which is lower than the 30 microgram doses being given to Pfizer vaccine recipients ages 12 years and older. (Earlier results had found increased side effects for the younger children at the higher dose.) Antibody responses and side effects in the 5-to-11-year-olds were compared to those of 16-to 25-year-old participants from a previous study.

The initial trial results showed that the antibody response in 5-to-11-year-olds was similar to the antibody response in teens and young adults. Side effects were also comparable.

A subsequent analysis compared COVID-19 infections in the vaccine and placebo groups. It found the Pfizer vaccine to be 90.7% effective at protecting against COVID-19 infection, compared to placebo. This was based on three cases of COVID-19 in vaccinated study participants, compared to 16 cases in the placebo group. There were no cases of severe COVID-19 or multisystem inflammatory syndrome in children (MIS-C) in either group during the study.

The FDA expanded its EUA for the Pfizer/BioNTech COVID-19 vaccine to include adolescents 12 to 15 years old in May 2021. Previously, the Pfizer vaccine was authorized for use in children 16 years and older. A CDC report published in MMWR in October 2021 showed that 12- to 18-year-olds who’d gotten two doses of the Pfizer vaccine were 93% less likely than those who were unvaccinated to be hospitalized for COVID-19. COVID-related hospitalizations in children and adolescents are still relatively rare; the majority of participants in this study had at least one underlying medical condition that put them at increased risk for severe illness.

The Moderna and Johnson & Johnson vaccines are currently authorized for people 18 years and older. 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.

Age de-escalation studies, in which the vaccines are tested in groups of children of descending age, 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 FDA reviews data from the de-escalation trials to decide whether to authorize the vaccines for each age group.

Does vaccination protect against long COVID?

Being fully vaccinated with a COVID-19 vaccine does reduce the risk of long COVID. Also called post-COVID-19 syndrome or post-acute sequelae of SARS-CoV-2 infection (PASC), long COVID is the term for symptoms that arise or persist more than 28 days after an initial COVID-19 infection.

Vaccination may reduce the risk of long COVID in two ways. The first is by reducing the risk of becoming infected with COVID in the first place. But what about people who experience breakthrough infections, becoming infected despite being vaccinated?

A study published in The Lancet shows that fully vaccinated people who experience breakthrough infections are about 50% less likely to develop long COVID than people who are infected without having been vaccinated. These results are based on self-reported data submitted via the COVID Symptom Study phone app in the United Kingdom between December 2020 and July 2021.

Click here for more new questions and answers.  

Symptoms, spread, and other essential information

What is coronavirus and how does it spread? What is COVID-19 and what are the symptoms? How long does coronavirus live on different surfaces? Take a moment to reacquaint yourself with basic information about this virus and the disease it causes.

Click here to read more about COVID-19 symptoms, spread, and other basic information.

Social distancing, hand washing, and other preventive measures

By now, many of us are taking steps to protect ourselves from infection. This likely includes frequent handwashing, regularly cleaning frequently touched surfaces, and social distancing. How do each of these measures help slow the spread of this virus, and is there anything else you can do?

Click here to read more about what you can do to protect yourself and others from coronavirus infection.

If you are at higher risk

Though no one is invulnerable, we've seen that older adults are at increased risk for severe illness or death from COVID-19. Underlying conditions, including heart disease, lung disease, and diabetes, increase risk even further in those who are older. In addition, anyone with an underlying medical condition, regardless of their age, faces increased risk of serious illness.

Click here to read more about what you can do if you are at increased risk for serious illness.

If you've been exposed, are sick, or are caring for someone with COVID-19

Despite your best efforts, you may be exposed to coronavirus and become ill with COVID-19. Or you may be in a position where you are caring for a loved one with the disease. It's important to know what to do if you find yourself in any of these situations. Stock up with medications and health supplies now, and learn the steps you can take to avoid infecting others in your household and to avoid getting sick yourself if you are caring for someone who is ill.

Click here to read more about what to do you if you have been exposed, are sick, or are caring for someone with COVID-19.

Treatments for COVID-19: What helps, what doesn't, and what's in the pipeline

While there are no specific treatments for COVID-19 at this time, there are things you can do to feel better if you become ill. In the meantime, researchers around the globe are looking at existing drugs to see if they may be effective against the virus that causes COVID-19, and are working to develop new treatments as well.

Click here to read more about measures that can help you feel better and treatments that are under investigation.

Coronavirus and kids

So far, the vast majority of coronavirus infections have afflicted adults. And when kids are infected, they tend to have milder disease. Still, as a parent, you can't help but worry about the safety of your children. Many parents are also trying to find a balance between answering their children's questions about the pandemic and enforcing health-promoting behaviors and social distancing rules without creating an atmosphere of anxiety. Not to mention keeping kids engaged and entertained with schools closed and playdates cancelled.

Click here to read more about kids and the coronavirus outbreak.

Coping with coronavirus

The news about coronavirus and its impact on our day-to-day lives has been unrelenting. There's reason for concern and it makes good sense to take the pandemic seriously. But it's not good for your mind or your body to be on high alert all the time. Doing so will wear you down emotionally and physically.

Click here to read more about coping with coronavirus.

More new questions and answers

What types of masks are most and least effective?

We know that wearing masks can help prevent the spread of coronavirus by blocking droplets, and smaller particles called aerosols, that are emitted when someone coughs, sneezes, talks, or breathes. But which masks are best and worst? For the general public, a properly fitted surgical mask is the best option, and should be worn whenever possible.

Researchers at Duke University created a simple setup that allowed them to count the number of droplet particles released when people spoke the phrase "Stay healthy, people" five times in a row. First, the study participants spoke without a mask, and then they repeated the same words, each time wearing one of 14 different types of face masks and coverings.

As expected, medical grade N95 masks performed best, meaning that the fewest number of droplets got through. They were followed by surgical masks. Several masks made of polypropylene, a cotton/propylene blend, and two-layer cotton masks sewn in different styles also performed well.

Gaiters ranked dead last. Also called neck fleeces, gaiters tend to be made of lightweight fabric and are often worn by athletes. Bandanas also ranked poorly.

Although the CDC recommends masks made of two or more layers of washable, breathable fabric, surgical masks are more effective than cloth masks at filtering out smaller particles. Regardless of the type of mask you wear, make sure it completely covers your nose and mouth and fits snugly against the sides of your face without leaving any gaps.

The CDC has information on how to improve the fit of your mask.

Is there a pill that can reduce my risk of being hospitalized if I get COVID-19?

In October 2021, Merck released promising study results about an oral antiviral drug to treat COVID-19. Compared to placebo, the antiviral drug, called molnupiravir, significantly reduced the risk of hospitalization and death in people with mild or moderate COVID-19 who were at high risk for severe COVID. If the FDA authorizes or approves this drug, molnupiravir will be the first COVID-19 treatment that can be taken by mouth early in the course of infection to reduce disease severity.

The study results were based on data from 775 study participants from the US and around the world. To be eligible for the study, the participants had to have been diagnosed with mild-to-moderate COVID-19, have started experiencing symptoms no more than five days prior to their enrollment in the study, and have at least one risk factor that put them at increased risk for a poor outcome from COVID-19. None of the participants were hospitalized at the time they entered the study. About half of the study participants took the antiviral drug molnupiravir; four capsules, twice a day, for five days, by mouth. The remaining study participants took a placebo.

Patients taking molnupiravir were half as likely to be hospitalized or die from COVID-19 as those taking a placebo. Over the 29-day study period, 28 out of 385, or 7.3%, of participants who took molnupiravir were hospitalized, and no one in this group died. In the placebo group, 53 out of 377, or 14.1%, of participants were hospitalized, including eight participants in this group who died. The antiviral drug was effective against several COVID variants, including the Delta variant.

The study was randomized, placebo-controlled, and double-blind — the gold standard in study design. The results were announced in a press release and have not been peer-reviewed or published in a medical journal. According to the press release, the risk of adverse events and drug side effects was comparable in the molnupiravir and placebo groups.

Molnupiravir was developed by Merck and Ridgeback Biotherapeutics. It works by interfering with the COVID virus’s ability to replicate.

Another drug, remdesivir (Veklury), can also reduce risk of hospitalization when given early in the course of infection. But remdesivir must be given intravenously in a medical setting.

The prospect of having an oral antiviral to combat COVID-19 is exciting, but is not a substitute for getting vaccinated. The COVID vaccine remains more important than ever. We need layers of defense against this viral threat. Hopefully molnupiravir will be part of that defense.

I recently spent time with someone who tested positive for COVID-19. I’m fully vaccinated. Do I need to get tested?

Yes, you do. In July 2021, the CDC recommended that anyone who is fully vaccinated and comes into contact with someone who has, or is suspected of having, COVID-19 should get tested three to five days after exposure. In addition, you should wear a mask in public indoor settings for 14 days or until you receive a negative test result. If you are vaccinated, you do not need to quarantine, but you should isolate if you develop symptoms or receive a positive test result.

Previously, the CDC had said that someone who was fully vaccinated only needed to get tested after exposure if they were experiencing symptoms. The change follows new evidence regarding the Delta variant, which shows that people who are vaccinated and then get infected (breakthrough infections) can spread the virus to others, perhaps to the same extent as those who are unvaccinated.

If you are not fully vaccinated, a 14-day quarantine remains the best way to avoid spreading the virus to others after you've been exposed to someone with COVID-19. According to CDC guidelines, you may discontinue quarantine after a minimum of 10 days if you do not have any symptoms, or after a minimum of seven days if you have a negative COVID test within 48 hours of when you plan to end quarantine.

What do I need to know about the Delta variant?

The Delta variant is different from previous SARS-CoV-2 variants that have made their way to the US. It’s highly contagious and may cause more severe disease. It is also more likely than previous variants to be spread by vaccinated people experiencing breakthrough infections.

Evidence presented to the CDC showed the Delta variant to be much more contagious than previous variants, with a risk of transmission similar to chickenpox. It also appears that people who are infected can spread the virus for longer periods of time.

In addition, people who are vaccinated and then get infected (breakthrough infections) can also spread the virus to others, perhaps to the same extent as those who are unvaccinated. This is also new; vaccinated people were much less likely to spread previous variants.

Finally, international studies point to the Delta variant being more likely to cause severe disease.

The good news is that while vaccination may be less effective at preventing infection and spread of the Delta variant, it is still highly protective against severe disease, hospitalization, and death. Get the COVID-19 vaccine if you have not already done so.

In July 2021, the CDC advised all people — vaccinated and unvaccinated — to wear masks in public indoor places, in areas of the country with substantial or high transmission of the virus. Given the threat posed by the Delta variant, it’s reasonable for everyone to start wearing a mask in indoor public spaces, regardless of the level of virus in your area.

Masks reduce the amount of virus we breathe in, and breathe out. Combined with the vaccine, masks provide a one-two punch that reduces the risk of spread — to children who are not yet eligible for vaccines, to people with weakened immune systems, and to others who are unvaccinated. Masks also provide additional protection for the wearer, including who are fully vaccinated.

Why is the CDC asking fully vaccinated people to wear masks again? Where and when do I need to wear a mask now?

In July 2021, the CDC advised all people — vaccinated and unvaccinated — to wear masks in public indoor places, in areas with substantial or high transmission of the virus. The CDC has always advised unvaccinated people to mask indoors, and also advises anyone at increased risk to wear a mask indoors, regardless of the level of community transmission. The change in guidance for people who are fully vaccinated was made amidst increasing numbers of infections and hospitalizations across the country.

One factor driving increased infections is the rise of the Delta variant, which spreads more easily than other variants. The Delta variant is now the dominant variant in the US.

We know that people who are fully vaccinated have a much smaller risk of getting sick if they are exposed the Delta variant. While they are also less likely to spread the virus, the Delta variant is more capable than the original virus of getting into cells that line the nose, mouth, and throat. Once these variants get inside the cells, they rapidly make copies of themselves, increasing what is called the viral load. That’s why people who are fully vaccinated can still carry greater amounts of the Delta variant, making it more likely that they could spread the virus to others.

This is where the new masking guidelines come in. Masks reduce the amount of virus we breathe in, and breathe out. Combined with the vaccine, masks provide a one-two punch that reduces the risk of spread — to children who are not yet eligible for vaccines, to people with weakened immune systems, and to others who are unvaccinated.

Masks also provide additional protection for the wearer, even those who are fully vaccinated. Vaccines have been very effective, even against the Delta variant, in protecting against severe infection, hospitalization, and death. However, the Delta variant has led to a number of breakthrough infections in people who are fully vaccinated, and masks provide another layer of protection.

To check the level of virus transmission in your area, visit the CDC’s COVID Data Tracker. Areas with substantial or high transmission appear in orange or red.

More about COVID-19

Reliable resources

Terms to know

aerosols: infectious viral particles that can float or drift around in the air. Aerosols are emitted by a person infected with coronavirus — even one with no symptoms — when they talk, breathe, cough, or sneeze. Another person can breathe in these aerosols and become infected with the virus. Aerosolized coronavirus can remain in the air for up to three hours. A mask can help prevent that spread.

antibodies: proteins made by the immune system to fight infections. If the antibodies later encounter the same infection, they help prevent illness by recognizing the microbe and preventing it from entering cells.

antibody test: also known as a serologic test, an antibody test is a blood test that looks for antibodies created by your immune system. An antibody test can indicate if you were previously infected but is not a reliable way to determine whether you are currently infected.

antigen: a substance displayed on the surface of a microbe that stimulates the body to produce an immune response.

antigen test: a diagnostic test that detects specific proteins on the surface of the virus.

booster: an additional dose of COVID-19 vaccine given after protection from the initial vaccine series begins to decline. A homologous booster is the same brand as the initial vaccine; a heterologous booster is a different brand than the initial vaccine.

community spread (community transmission): is said to have occurred when people have been infected without any knowledge of contact with someone who has the same infection

contact tracing: a process that begins with identifying everyone a person diagnosed with a given illness (in this case COVID-19) has been in contact with since they became contagious. The contacts are notified that they are at risk, and may include those who share the person's home, as well as people who were in the same place around the same time as the person with COVID-19 — a school, office, restaurant, or doctor's office, for example. Contacts may be quarantined or asked to isolate themselves if they start to experience symptoms, and are more likely to be tested for coronavirus if they begin to experience symptoms.

containment: refers to limiting the spread of an illness. Because no vaccines exist to prevent COVID-19 and no specific therapies exist to treat it, containment is done using public health interventions. These may include identifying and isolating those who are ill, and tracking down anyone they have had contact with and possibly placing them under quarantine.

diagnostic test: indicates whether you are currently infected with COVID-19. A sample is collected using a swab of your nose, your nose and throat, or your saliva. The sample is then checked for the virus's genetic material (PCR test) or for specific viral proteins (antigen test).

effectiveness: indicates the benefit of a vaccine in the real world.

efficacy: indicates the benefit of a vaccine compared to a placebo in the context of a clinical trial.

epidemic: a disease outbreak in a community or region

flattening the curve: refers to the epidemic curve, a statistical chart used to visualize the number of new cases over a given period of time during a disease outbreak. Flattening the curve is shorthand for implementing mitigation strategies to slow things down, so that fewer new cases develop over a longer period of time. This increases the chances that hospitals and other healthcare facilities will be equipped to handle any influx of patients.

false negative: a test result that mistakenly indicates you are not infected when you are.

false positive: a test result that mistakenly indicates you are infected when you are not.

herd immunity: herd immunity occurs when enough people become immune to a disease to make its spread unlikely. As a result, the entire community is protected, even those who are not themselves immune. Herd immunity is usually achieved through vaccination, but it can also occur through natural infection.

immunity: partial or complete protection from a specific infection because a person has either had that infection previously or has been vaccinated against it.

incubation period: the period of time between exposure to an infection and when symptoms begin

isolation: the separation of people with a contagious disease from people who are not sick

long-haulers: people who have not fully recovered from COVID-19 weeks or even months after first experiencing symptoms.

mitigation: refers to steps taken to limit the impact of an illness. Because no vaccines exist to prevent COVID-19 and no specific therapies exist to treat it, mitigation strategies may include frequent and thorough handwashing, not touching your face, staying away from people who are sick, social distancing, avoiding large gatherings, and regularly cleaning frequently touched surfaces and objects at home, in schools, at work, and in other settings.

mutation: A change to a virus’s genetic material that occurs when the virus is replicating. The change is passed on to future generations of the virus.    

monoclonal antibodies: laboratory-produced proteins designed to mimic naturally occurring antibodies that target specific antigens on viruses, bacteria, and cancer cells.  

mRNA: short for messenger ribonucleic acid, mRNA is genetic material that contains instructions for making proteins.

mRNA vaccines: 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 the COVID-19 virus. 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.

pandemic: a disease outbreak affecting large populations or a whole region, country, or continent

physical distancing: also called social distancing, refers to actions taken to stop or slow down the spread of a contagious disease. For an individual, it refers to maintaining enough physical distance (a minimum of six feet) between yourself and another person to reduce the risk of breathing in droplets or aerosols that are produced when an infected person breathes, talks, coughs, or sneezes.

polymerase chain reaction (PCR) test: a diagnostic test that detects the presence of the virus's genetic material.

post-viral syndrome: the constellation of symptoms experienced by COVID-19 long haulers. These symptoms may include fatigue, brain fog, shortness of breath, chills, body ache, headache, joint pain, chest pain, cough, and lingering loss of taste or smell.

presumptive positive test result: a positive test for the virus that causes COVID-19, performed by a local or state health laboratory, is considered "presumptive" until the result is confirmed by the CDC. While awaiting confirmation, people with a presumptive positive test result will be considered to be infected.

quarantine: separates and restricts the movement of people who have a contagious disease, have symptoms that are consistent with the disease, or were exposed to a contagious disease, to see if they become sick

SARS-CoV-2: short for severe acute respiratory syndrome coronavirus 2, SARS-CoV-2 is the official name for the virus responsible for COVID-19.

social distancing: also called physical distancing, refers to actions taken to stop or slow down the spread of a contagious disease. For an individual, it refers to maintaining enough physical distance (a minimum of six feet) between yourself and another person to reduce the risk of breathing in droplets or aerosols that are produced when an infected person breathes, talks, coughs, or sneezes. It is possible to safely maintain social connections while social distancing, through phone calls, video chats, and social media platforms.

spike protein: a protein on the surface of the SARS-CoV-2 virus that binds to and allows the virus to enter human cells.

variant: A virus containing one or more mutations that make it different from a version of the virus that has been circulating.

variants of concern: SARS-CoV-2 viruses with mutations that make them more likely to spread, evade vaccines, or make people sicker.

vector: a harmless capsule. In a vaccine, a vector may be used to deliver a substance into the body in order to prompt an immune response.

virus: a virus is the smallest of infectious microbes, smaller than bacteria or fungi. A virus consists of a small piece of genetic material (DNA or RNA) surrounded by a protein shell. Viruses cannot survive without a living cell in which to reproduce. Once a virus enters a living cell (the host cell) and takes over a cell's inner workings, the cell cannot carry out its normal life-sustaining tasks. The host cell becomes a virus manufacturing plant, making viral parts that then reassemble into whole viruses and go on to infect other cells. Eventually, the host cell dies.

Image: Naeblys/Getty Images


Harvard Health Publishing Coronavirus Resource Center Experts

The Harvard Health Publishing team would like to acknowledge the Harvard Medical School experts who have contributed their time and expertise: Steven A. Adelman, MDAshwini Bapat, MD; Nicole Baumer, MD, MEd; Suzanne Bertisch, MD, MPHJoseph R. Betancourt, MD, MPHBarry R. Bloom, PhDEmeric Bojarski, MDMelissa Brodrick, MEdAndrew E. Budson, MDStephanie Collier, MD, MPHTodd Ellerin, MDHuma Farid, MD; Elizabeth Pegg Frates, MD; Robert Gabbay, MD, PhD, FACPAlan Geller, MPH, RNEllen S. Glazer, LICSW; David C. Grabowski, PhD; Shelly Greenfield, MD, MPH; Ilona T. Goldfarb, MD, MPH; Peter Grinspoon, MDAbraar Karan, MD, MPH, DTM&HSabra L. Katz-Wise, PhDAlyson Kelley-Hedgepeth, MDAnthony Komaroff, MDDouglas Krakower, MDDebi LaPlante, PhDHoward E. LeWine, MDDara K. Lee Lewis, MDSharon Levy, MD, MPHKristina Liu, MD, MHSJulia Marcus, PhD, MPHLuana Marques, PhDClaire McCarthy, MDChris McDougle, MDBabar Memon, MD, MSc; Kristin Moffitt, MD; Uma Naidoo, MDJanelle Nassim, MDJustin NeimanVikram Patel, MBBS, PhDEdward Phillips, MDShiv Pillai, PhD, MBBSJohn Ross, MD, FIDSALee H. Schwamm, MDCatherine Ullman Shade, PhD, MEdHoward J. Shaffer, PhD, CASRoger Shapiro, MD, MPHJohn Sharp, MD; Amy C. Sherman, MD; Robert H. Shmerling, MDJacqueline Sperling, PhDFatima Cody Stanford, MD, MPH, MPA, FAAP, FACP, FTOS; Dawn Sugarman, PhD; Monique Tello, MD, MPHRobyn Thom, MDKaren Turner, OTR/LRochelle Wallensky, MD, MPHJanice Ware, PhD; Bobbi Wegner, PsyD; Scott Weiner, MDSarah Wilkie, MSAnna R. Wolfson, MD.


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.

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