Coronavirus Resource Center

Helping you keep up with the latest as we continue to learn more about coronavirus and COVID-19

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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

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 did the FDA and CDC pause, and then resume, 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 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 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 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.

How have the vaccines performed in 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 3,950 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.

I'm vaccinated. What can I do now?

In April 2021, the CDC released guidance that fully vaccinated people no longer need to wear a mask outdoors, except in certain crowded settings and venues. All outdoor activities are considered safe for fully vaccinated people, with continued masks and distancing only advised for crowded outdoor activities.

Vaccines offer excellent (though not complete) protection against moderate to severe disease, hospitalization, and death. Early evidence suggests you're also less likely to infect others once you've been vaccinated.

Many indoor activities are also considered safe for fully vaccinated people, with the caveat that even those who have been vaccinated should continue to wear masks and physically distance in indoor spaces, like malls or grocery stores, where there is likely to be a mix of vaccinated and unvaccinated people.

The CDC says that small indoor gatherings with others who are also fully vaccinated are okay, and do not require masks or distancing. Many people are eager to visit older family members; it'll be much safer to do so once you're all vaccinated.

Fully vaccinated people may consider gathering indoors with unvaccinated people from a single household who are at low risk for severe COVID-19. It's safest to keep your mask on, and when weather permits, why not meet outdoors, or at least keep the windows open? For now, it's still best to avoid medium to large indoor gatherings.

Slowly but surely, we are returning to the things we miss.

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

You've gotten the basics down: you're wearing your mask, avoiding crowds, and keeping your distance from friends and family. 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.


COVID-19 vaccines

Based on strong scientific evidence of safety and effectiveness, the FDA authorized the first COVID-19 vaccines less than one year after the first COVID-19 cases were reported. You may be excited, but cautious. You likely still have questions. How do the vaccines work? Who should get them? How long will protection last? Will the vaccine prevent me from infecting others?

Click here to learn more about COVID-19 vaccines.


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

Most people who become ill with COVID-19 can recover at home, and there are things you can do to feel better. The FDA has also authorized treatments that doctors may use to treat hospitalized COVID-19 patients, and to treat COVID-19 patients who are not hospitalized but are at risk for becoming severely ill. Researchers around the globe continue working hard to develop other effective treatments.

Click here to read more about measures that can help.


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 outdoor and indoor activities are considered safe? Does it matter if I've been vaccinated? And do I still need to wear a mask?

According to new guidance from the CDC, fully vaccinated people no longer need to wear a mask outdoors, except in certain crowded settings and venues. This was part of a larger CDC announcement that outlines the relative safety of indoor and outdoor activities for vaccinated and unvaccinated people, and provides guidance on mask wearing and other preventive measures.

In broad strokes, all outdoor activities are considered safe for fully vaccinated people, with continued masks and distancing only advised for crowded outdoor activities. Many indoor activities are also considered safe for fully vaccinated people, with the caveat that even those who have been vaccinated should continue to wear masks and physically distance in indoor spaces, like malls, where there are likely to be a mix of vaccinated and unvaccinated people. (Earlier CDC guidance had already made it okay for small groups of fully vaccinated people to gather indoors without masks or distancing.)

Unvaccinated people may also choose to go unmasked while walking, running, or biking outdoors or when gathering outdoors with a small group of vaccinated people. However, unvaccinated people are advised to mask in other outdoor situations. They should also mask and distance indoors, and should be aware that all indoor activities are considered less safe for people who have not been fully vaccinated.

The CDC has created a helpful chart illustrating what is safe to do, with which prevention measures, and depending on your vaccination status. To view the chart, click here.

Is it safe to travel once I'm vaccinated? What if I'm not vaccinated?

Guidance from the CDC issued in April 2021 states that fully vaccinated people may travel more freely within the US. Travelers do not need to get COVID testing before or after travel and do not need to quarantine, unless required by local or state authorities.

The CDC is more cautious about international travel, noting the increased risk of variants in other countries, even for people who are fully vaccinated.

As a general rule, travel can increase your chance of spreading and getting COVID-19 if you are not vaccinated. The CDC recommends that unvaccinated travels get tested before and after domestic travel and self-quarantine for seven to 10 days after travel. They discourage unvaccinated people from travelling outside of the US.

All travelers should wear a mask that covers the nose and mouth, maintain a physical distance of six feet from others, avoid crowds, and wash hands often. Anyone who is sick or has tested positive for COVID-19 should not fly if at all possible.

Stay current on travel advisories from regulatory agencies.

What do we know about new COVID-19 variants that originated in the United States?

New viral variants occur all the time, the result of mistakes, or mutations, made when the virus is replicating. Two new variants of the SARS-CoV-2 virus have emerged, one that originated in New York, and the other in California. They are considered to be "variants of concern" because of early evidence that they may be more contagious than other forms of the virus, and less vulnerable to immune responses generated by vaccines or previous infection.

The New York variant, called B.1.526, actually refers to two versions of mutated virus that have been grouped together under a single name. One version has the E484K mutation. This mutation also appears in the South African (B.1.351) variant and the Brazil (P.1) variant, and several studies have shown that variants containing the E484K variant are less susceptible to vaccines. The other version of the variant has a mutation on its spike protein that may affect how tightly the virus can bind to human cells. Early laboratory evidence suggests that antibodies created in response to both the Pfizer/BioNTech and Moderna mRNA vaccines are effective against the New York variant.

The California variant is called B.1.427/B.1.429, and it is now the dominant variant in California. Scientists have observed that people infected with this variant have twice as many viral particles in their noses; a higher viral load can mean increased transmissibility.

Every new variant raises the same concerns:

  • Is it more contagious?
  • Will vaccines be less effective?
  • Can they re-infect people who were already infected with a different variant?

While we don't know the answers to all of these questions, we do know that mask wearing, physical distancing, avoiding crowds, and getting vaccinated once you are eligible continue to be the best ways to protect yourself and others.

What needs to happen for schools to open safely?

School closures have impacted children on many fronts, from academics and social interaction to equity, food security, and mental health. The CDC recently released guidelines for opening schools, noting that decisions about when and how to safely open schools should consider levels of community transmission.

The guidelines prioritize five key mitigation strategies, which, when layered together, should minimize the spread of COVID-19 in schools:

  • universal, correct use of masks that cover the mouth and nose
  • physical distancing
  • frequent hand washing
  • cleaning high-touch surfaces and maintaining healthy facilities
  • contact tracing, in combination with isolation and quarantine.

The CDC guidelines for physical distancing vary by school level and degree of community transmission, and they assume that everyone is masked. The CDC recommends at least 3 feet between students in elementary, middle, and high schools when community transmission is low or moderate. When transmission is high, students should be divided into separate groups (cohorts) that maintain 6 feet of distance from other cohorts throughout the day. The CDC also recommends a minimum of 6 feet between teachers and students at all times, and a minimum distance of 6 feet when masks are off, such as during lunch time.

The guidelines also recommend improving ventilation, by opening windows and doors, for example. Ideally, schools should have access to testing for students and teachers with symptoms, as well as routine screening to identify asymptomatic cases. And while they acknowledge that vaccination of teachers is important, they do not require it before returning to school.

Lately I've seen more people wearing two masks. Should I be double masking?

According to new guidance from the CDC, yes you should. The guidance is based on a lab study, published in MMWR, which used masked and unmasked dummies that released aerosol particles from a mouthpiece when they were simulated to cough or breathe. The study found that wearing a multilayered cloth mask over a surgical mask or wearing a tightly fitted surgical mask substantially increased the level of protection for both the mask wearer and others.

When double masking, the CDC recommends wearing a snug cloth mask over a surgical mask. Surgical masks provide better filtration, but tend to fit loosely. Cloth masks close any gaps and provide another layer of protection. Surgical masks are sometimes called medical masks or medical procedure masks.

Adjusting a surgical mask for a tighter fit using a method called "knotting and tucking" also offers good protection. To knot and tuck a surgical mask, knot the ear loops of a 3-ply face mask where they join the edge of the mask, then fold and tuck the unneeded material under the edges. For video instructions on how to knot and tuck a surgical mask, click here. Mask fitters, or mask braces, which are worn over a cloth or surgical mask, can also improve mask fit.

In the CDC's lab study, double masking or tight-fitting surgical masks reduced both transmission of and exposure to aerosols by about 95% compared to no masking.

Do the newer COVID-19 variants spread more easily? And are they more dangerous?

Like other viruses, the coronavirus responsible for COVID-19 — SARS-CoV-2 — cannot survive without a living cell in which to reproduce. Once it enters human cells, SARS-CoV-2 churns out copies of itself, which go on to infect other cells. Sometimes, a mistake is made when the virus is replicating. This is called a mutation.

Mutations have led to at least three new, concerning coronavirus variants. One, called B.1.1.7, was first detected in the United Kingdom. Another, called B.1.351, originated in South Africa. A third, called P.1 originated in Brazil. All three variants have now been detected in countries around the globe.

All of these variants contain mutations on the virus's spike protein. Spike proteins on the surface of the SARS-CoV-2 virus bind to and allow the virus to enter human cells. All three variants share a key mutation (called N501Y) on the spike protein, which allows the virus to bind more tightly to human cells. This mutation makes the new variants more contagious than previous SARS-CoV-2 variants. The B.1.351 and P.1 variants also share a second mutation, called E484K, which might make it easier for them to re-infect someone who has already been infected or, possibly, someone who has been vaccinated.

The B.1.1.7 variant is estimated to be about 50% more transmissible than previous SARS-CoV-2 variants. On April 7, 2021, the CDC announced that B.1.1.7 is now the most common variant circulating in the US. There are conflicting reports about whether this variant is more virulent; two published studies found that the B.1.1.7 variant is associated with a 55% to 64% greater chance of severe illness and death, but another study, published in The Lancet, did not find this variant to be associated with an increased risk of severe disease or death. Existing COVID-19 vaccines appear to be effective against the B.1.1.7 variant.

The emergence of these highly transmissible variants is yet another reason why mask wearing, physical distancing, avoiding crowds, and getting vaccinated once you are eligible continue to be as important as ever.


More about COVID-19


Video: How does an mRNA vaccine work?

Video Credit: Justin Saglio/Harvard Public Affairs & Communications



Reliable resources

Interested in participating in COVID-19 research by tracking daily symptoms through a simple app? Help doctors and scientists at Massachusetts General Hospital and Harvard T.H. Chan School of Public Health study the symptoms of COVID-19 and track the spread of this virus by downloading the COVID Symptom Study app.


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.

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.

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.

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.

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


Questions?

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, MD; Ashwini Bapat, MD; Suzanne Bertisch, MD, MPH; Joseph R. Betancourt, MD, MPH; Barry R. Bloom, PhD; Emeric Bojarski, MD; Melissa Brodrick, MEd; Andrew E. Budson, MD; Stephanie Collier, MD, MPH; Todd Ellerin, MD; Huma Farid, MD; Robert Gabbay, MD, PhD, FACP; Alan Geller, MPH, RN; Ellen S. Glazer, LICSW; Ilona T. Goldfarb, MD, MPH; Peter Grinspoon, MD; Abraar Karan, MD, MPH, DTM&H; Sabra L. Katz-Wise, PhD; Alyson Kelley-Hedgepeth, MD; Anthony Komaroff, MD; Douglas Krakower, MD; Debi LaPlante, PhD; Howard E. LeWine, MD; Dara K. Lee Lewis, MD; Sharon Levy, MD, MPH; Kristina Liu, MD, MHS; Julia Marcus, PhD, MPH; Luana Marques, PhD; Claire McCarthy, MD; Chris McDougle, MD; Babar Memon, MD, MSc; Uma Naidoo, MD; Janelle Nassim, MD; Justin Neiman; Vikram Patel, MBBS, PhD; Edward Phillips, MD; Shiv Pillai, PhD, MBBS; John Ross, MD, FIDSA; Lee H. Schwamm, MD; Catherine Ullman Shade, PhD, MEd; Howard J. Shaffer, PhD, CAS; Roger Shapiro, MD, MPH; John Sharp, MD; Robert H. Shmerling, MD; Jacqueline Sperling, PhD; Fatima Cody Stanford, MD, MPH, MPA, FAAP, FACP, FTOS; Monique Tello, MD, MPH; Robyn Thom, MD; Karen Turner, OTR/L; Rochelle Wallensky, MD, MPH; Janice Ware, PhD; Bobbi Wegner, PsyD; Scott Weiner, MD; Sarah Wilkie, MS; Anna R. Wolfson, MD.

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.