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Coronavirus Resource Center

June 10, 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 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.

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.

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.

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.

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.

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

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

Mutations have led to at least four concerning coronavirus variants. One, called Alpha, or B.1.1.7, was first detected in the United Kingdom. Another, called Beta, or B.1.351, originated in South Africa. A third, called Gamma, or P.1, originated in Brazil. And a fourth, called Delta, or B.1.617.2, was first documented in India. All four 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. The Alpha, Beta, and Gamma 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 Beta and Gamma 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. Two peer-reviewed studies published in May 2021 found the Pfizer/BioNTech vaccine to be extremely effective against the B.1.1.7 and the B.1.351 variants.

The Delta, or B.1.617.2, variant is prevalent in India and is also now the dominant variant in the United Kingdom. It is highly transmissible, and there is some evidence that it is associated with increased disease severity. However, a study from Public Health England found that full vaccination with the Pfizer/BioNTech vaccine was 88% effective against this variant.

The emergence of these more transmissible and virulent variants is yet another reason why getting vaccinated once you are eligible and following CDC guidance on prevention measures continue to be as important as ever.

I had COVID-19 a few months ago. Am I at increased risk for health problems in the future?

It does appear that people who recover from COVID-19 have an increased risk of developing other medical conditions, at least in the short term.

One study, published in The BMJ, collected laboratory test and hospital admissions data from a health plan in the US. The researchers compared data from more than 190,000 adults, ages 18 to 65 years, who tested positive for the SARS-CoV-2 virus in 2020, to data from a control group that was collected in 2019, before the pandemic. The researchers followed the participants for six months after they tested positive for SARS-CoV-2 and recorded any new health complications.

They found that 14% of people who had had COVID-19 developed a new medical issue during the following six months; this was nearly 5% higher than the pre-pandemic control group, a significant difference. New medical problems affected a range of body systems and included respiratory failure, abnormal heart rhythms, diabetes, neurological problems, and liver and kidney problems. Increased risk was seen in younger, previously healthy people, but was higher in older people and those with pre-existing medical problems.

Another study, published in Nature, compared the health records of more than 73,000 users of the Veteran’s Health Administration (VHA) who tested positive for SARS-CoV-2 but were never hospitalized, to those of nearly five million other VHA users who never tested positive for COVID-19 and were never hospitalized. For six months following the first 30 days after infection, people who had had COVID-19 were significantly more likely to die or to experience a medical or mental health problem that they had never had before.

These studies provide yet another reason to get a COVID-19 vaccine if you are eligible.

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 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 outdoor settings and in most indoor settings.

If you have a weakened immune system due to a medical condition or to medications you are taking, you may be less protected by the vaccine and thus at increased risk when you are around others who have not been vaccinated. Talk to your doctor to find out what safety measures you should continue to take.

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.

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 7 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. Variants may be considered to be "variants of concern" if they are thought to 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. This variant has certain mutations in common with the South African (B.1.351) variant and the Brazil (P.1) variant, which is one of the reasons it was considered a variant of concern when it first emerged. However, evidence released by the CDC in May 2021 shows that in those who are vaccinated, the B.1.526 variant is not more likely to cause infection, and does not lead to more serious disease.

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 getting vaccinated once you are eligible, and mask wearing, physical distancing, and avoiding crowds if you are not vaccinated, 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, called 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.


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.

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


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, 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, MDRobert 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.

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.