Preventing the spread of the coronavirus

Physical distancing, masks, vaccines, 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. But you likely still have questions. Does wearing a mask protect you, others, or both? How exactly will physical distancing help? And what do you need to know about the new COVID-19 vaccines?

What can I do to protect myself and others from COVID-19?

The following actions help prevent the spread of COVID-19, as well as other coronaviruses and influenza:

  • Wear a face mask.
  • Maintain at least six feet of distance between yourself and others.
  • Avoid large gatherings.
  • Socialize outdoors.
  • Avoid close contact with people who are sick.
  • Minimize touching your eyes, nose, and mouth.
  • Stay home when you are sick.
  • Cover your cough or sneeze with a tissue, then throw the tissue in the trash.
  • Clean and disinfect frequently touched objects and surfaces regularly.
  • Wash your hands often with soap and water.

This chart illustrates how protective measures such as limiting travel, avoiding crowds, social distancing, and thorough and frequent handwashing can slow down the development of new COVID-19 cases and reduce the risk of overwhelming the health care system.

What do I need to know about washing my hands effectively?

Wash your hands often with soap and water for at least 20 seconds, especially after going to the bathroom; before eating; after blowing your nose, coughing, or sneezing; and after handling anything that's come from outside your home.

  • If soap and water are not readily available, use an alcohol-based hand sanitizer with at least 60% alcohol, covering all surfaces of your hands and rubbing them together until they feel dry.
  • Always wash hands with soap and water if hands are visibly dirty.
  • The CDC's handwashing website has detailed instructions and a video about effective handwashing procedures.

How does coronavirus spread?

The coronavirus spreads mainly from person to person. This can happen between people who are in close contact with one another. Droplets that are produced when an infected person coughs or sneezes may land in the mouths or noses of people who are nearby, or possibly be inhaled into their lungs.

A person infected with coronavirus — even one with no symptoms — may emit aerosols when they talk or breathe. Aerosols are infectious viral particles that can float or drift around in the air for up to three hours. Another person can breathe in these aerosols and become infected with the coronavirus. This is why everyone should wear a mask when they go out in public.

Coronavirus can also spread from contact with infected surfaces or objects, though this is a less common way for the virus to spread.

How could contact tracing help slow the spread of COVID-19?

Anyone who comes into close contact with someone who has COVID-19 is at increased risk of becoming infected themselves, and of potentially infecting others. Contact tracing can help prevent further transmission of the virus by quickly identifying and informing people who may be infected and contagious, so they can take steps to not infect others.

Contact tracing begins with identifying everyone that a person recently diagnosed with COVID-19 has been in contact with since they became contagious. In the case of COVID-19, a person may be contagious 48 to 72 hours before they started to experience symptoms.

The contacts are notified about their exposure. They may be told what symptoms to look out for, advised to isolate themselves for a period of time, and to seek medical attention as needed if they start to experience symptoms.

What is physical distancing and why is it important?

The COVID-19 virus primarily spreads when one person breathes in droplets or aerosols that are produced when an infected person coughs, sneezes, talks, or breathes.

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 distance (6 feet or more) between yourself and another person to avoid getting infected or infecting someone else. School closures, directives to work from home, library closings, and cancelling meetings and larger events help enforce physical distancing at a community level.

Slowing down the rate and number of new coronavirus infections is critical to reduce the risk that large numbers of critically ill patients cannot receive life-saving care.

What does the CDC's new definition of "close contacts" mean for me?

The CDC has expanded how it defines close contacts of someone with COVID-19. Until this point, the CDC had defined a close contact as someone who spent 15 or more consecutive minutes within six feet of someone with COVID-19. According to the new definition, a close contact is someone who spends 15 minutes or more within six feet of a person with COVID-19 over a period of 24 hours.

Close contacts are at increased risk of infection. When a person tests positive for COVID-19, contact tracers may identify their close contacts and urge them to quarantine to prevent further spread. Based on the new definition, more people will now be considered close contacts.

Many factors can affect the chances that infection will spread from one person to another. These factors include whether or one or both people are wearing masks, whether the infected person is coughing or showing other symptoms, and whether the encounter occurred indoors or outdoors. Though the "15 minutes within six feet rule" is a helpful guideline, it's always best to minimize close interactions with people who are not members of your household.

The CDC's new definition was influenced by a case described in the CDC's Morbidity and Mortality Weekly Report in which a correctional officer in Vermont is believed to have been infected after being within six feet for 17 non-consecutive minutes of six asymptomatic individuals, all of whom later tested positive for COVID-19.

What types of medications and health supplies should I have on hand for an extended stay at home?

Try to stock at least a 30-day supply of any needed prescriptions. If your insurance permits 90-day refills, that's even better. Make sure you also have over-the-counter medications and other health supplies on hand.

Medical and health supplies

  • prescription medications
  • prescribed medical supplies such as glucose and blood-pressure monitoring equipment
  • fever and pain medicine, such as acetaminophen
  • cough and cold medicines
  • antidiarrheal medication
  • thermometer
  • fluids with electrolytes
  • soap and alcohol-based hand sanitizer
  • tissues, toilet paper, disposable diapers, tampons, sanitary napkins
  • garbage bags.

What precautions can I take when grocery shopping?

The coronavirus that causes COVID-19 is primarily transmitted through droplets containing virus, or through viral particles that float in the air. The virus may be breathed in directly and can also spread when a person touches a surface or object that has the virus on it and then touches their mouth, nose, or eyes. There is no current evidence that the COVID-19 virus is transmitted through food.

Safety precautions help you avoid breathing in coronavirus or touching a contaminated surface and touching your face.

In the grocery store, maintain at least six feet of distance between yourself and other shoppers. Wipe frequently touched surfaces like grocery carts or basket handles with disinfectant wipes. Avoid touching your face. Wearing a cloth mask helps remind you not to touch your face and can further help reduce spread of the virus. Use hand sanitizer before leaving the store. Wash your hands as soon as you get home.

If you are older than 65 or at increased risk for any reason, limit trips to the grocery store. Ask a neighbor or friend to pick up groceries and leave them outside your house. See if your grocery store offers special hours for older adults or those with underlying conditions. Or have groceries delivered to your home.

Which precautions are really necessary when it comes to preventing COVID-19?

Wear masks. Physically distance. Socialize outdoors. Avoid crowded indoor spaces. Wash your hands frequently. These are essential precautions you should take to reduce your risk of catching or spreading coronavirus.

But what about some other precautions you may be taking? Do they help, or is it okay to let them go? Let's take a look.

You don't need to wear gloves when running errands. It's true that a person can get infected if they touch a surface or object that has viral particles on it, then touch their mouth, nose, or eyes. But this is not the main way the virus spreads. What's more, gloves won't prevent this type of transmission, and may even make it more likely that you will touch your face. Instead, wash your hands before you leave the house, use hand sanitizer when you're out and about, and wash your hands again when you get back home. In between, try to avoid touching your face.

You don't need to disinfect groceries or takeout containers. The risk of infection from food or food packaging is very small. The CDC advises against using disinfectant intended for hard surfaces on cardboard or other grocery items, which can absorb the chemicals. If you are concerned about takeout, transfer food to your own serving dishes. And wash your hands and disinfect your counters after putting away your groceries or handling takeout containers.

Your mail can come out of quarantine. Mail does not pose a large risk, and putting your mail aside for several days before opening it is unnecessary. Once again, your best bet is to wash your hands after handling anything you bring in from outside.

You can invite your cleaners back into your home. If you've asked your cleaners to come back, request that they wear masks the entire time they are in your home. Leave the house while the cleaners are present if you can; otherwise, move to a different part of the house and close the door. Open windows to increase airflow throughout the house.

It's okay to go for a swim. Pools are relatively safe, because the coronavirus doesn't spread through water and chlorine is a disinfectant. Try to maintain physical distancing in crowded pools, but don't wear cloth masks in the water: they are difficult to breathe through when wet. Do wear a mask when changing, however, and minimize time in changing rooms, which are often crowded and not well ventilated.

What can I do when social distancing?

Try to look at this period of social distancing as an opportunity to get to things you've been meaning to do.

Though you might be avoiding the gym right now, that doesn't mean you can't exercise. Take long walks or run outside (do your best to maintain at least six feet between you and non-family members when you're outside). Do some yoga or other indoor exercise routines when the weather isn't cooperating.

Kids need exercise too, so try to get them outside every day for walks or a backyard family soccer game (remember, this isn't the time to invite the neighborhood kids over to play).

Pull out board games that are gathering dust on your shelves. Have family movie nights. Catch up on books you've been meaning to read, or do a family read-aloud every evening.

It's important to stay connected even though we should not do so in person. Keep in touch virtually through phone calls, Skype, Zoom, video, and other social media. Enjoy a leisurely chat with an old friend you've been meaning to call.

If all else fails, go to bed early and get some extra sleep!

Should I wear a face mask?

The CDC now recommends that all adults and children over 2 years wear a mask when going out in public.

The coronavirus that causes COVID-19 is primarily transmitted through droplets containing virus, or through viral particles that float in the air. Even people who are infected but do not have symptoms, or have not yet developed symptoms, can infect others. Masks help minimize spread. They are to be used in additon to, not instead of, physical distancing.

What kind of mask should you wear? The CDC recommends masks made of two or more layers of washable, breathable fabirc. Surgical masks are also effective, but pay attention to whether there is enough supply for front-line workers and emergency responders in your community. Make sure the mask completely covers your nose and mouth and fits snugly against the sides of your face without leavingt any gaps.

While N95 masks are the most effective, these medical-grade masks should be reserved for healthcare workers.

Surgical masks are preferred if you are caring for someone who has COVID-19.

The CDC has information on how to make, wear, and clean nonsurgical masks.

The WHO offers videos and illustrations on when and how to use a mask.

What types of masks are most and least effective?

We know that wearing masks can help prevent the spread of coronavirus by blocking droplets that are emitted when someone coughs, sneezes, talks, or breathes. But which masks are best and worst?

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 2-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.

A couple of simple tests can help you gauge the effectiveness of your mask: If you can see through your mask when you hold it up to the light, or can breathe through it easily, it's probably not doing much to prevent spread.

Who do masks protect: the wearer, others, or both?

We've known for some time that masks help prevent people from spreading the coronavirus to others. Based on an analysis of existing information, a new study contends that masks may also protect mask wearers from becoming infected themselves.

Different masks, writes the study author, block viral particles to varying degrees. If masks lead to lower "doses" of virus being inhaled, then fewer people may become infected, and those who do may have milder illness.

Researchers in China experimented with hamsters to test the effect of masks. They put healthy hamsters and hamsters infected with SARS-CoV-2 (the COVID-19 coronavirus) in a cage, and separated some of the healthy and infected hamsters with a barrier made of surgical masks. Many of the "masked" healthy hamsters did not get infected, and those who did got less sick than previously healthy "maskless" hamsters.

A similar experiment cannot ethically be done in humans. But researchers have studied doses of flu virus and found that people who inhaled a higher dose of flu virus were more likely to get sick and experience symptoms. Observations of coronavirus outbreaks in processing plants and on cruise ships also support the idea that masks may help protect mask wearers.

Without more research, we can't be certain that masks protect the wearer. But we do know they don't hurt, and that they protect others.

Could wearing masks prevent COVID deaths?

According to a study published in the journal Nature Medicine, widespread use of masks could prevent nearly 130,000 of 500,000 COVID-related deaths estimated to occur by March 2021.

These numbers are based on an epidemiological model. The researchers considered, state by state, the number of people susceptible to coronavirus infection, how many get exposed, how many then become infected (and infectious), and how many recover. They then modeled various scenarios, including mask wearing, assuming that social distancing mandates would go into effect once the number of deaths exceeded 8 per 1 million people.

Modeling studies are based on assumptions, so the exact numbers are less important than the comparisons of different scenarios. In this study, a scenario in which 95% of people always wore masks in public resulted in many fewer deaths compared to a scenario in which only 49% of people (the self-reported national average of mask wearers) always wore masks in public.

This study reinforces the message that we can help prevent COVID deaths by wearing masks.

If I want to visit friends and family, does it matter whether we meet indoors or outdoors?

You are better off meeting friends and family outdoors. We know that coronavirus spreads when someone breathes in virus that an infected person emits through coughs or sneezes, or when they talk or breathe. Research has shown that in a confined, laboratory setting, droplets containing viral particles can remain afloat for eight to 14 minutes. Smaller infectious viral particles, called aerosols, can drift around in the air even longer.

Outdoors, air currents are more likely to scatter and dilute the virus, making transmission less likely than in a home, office, or other confined space with limited air circulation. Even outdoors, however, it's important to maintain a physical distance of at least six feet and wear a mask, to reduce risk even further.

Coronavirus also spreads when a person touches a contaminated surface and then touches their eyes, nose, or mouth. If you are participating in an outdoor gathering, bring your own foods, drinks, plates, and utensils.

Can the COVID-19 virus spread through air conditioning?

We don't know for certain if the COVID-19 virus spreads through air conditioning. But we do know that when it's hot and humid, people are more likely to stay indoors, with the windows closed — giving the virus more opportunity to spread.

Coronavirus spreads through droplets that an infected person emits through coughs or sneezes and through smaller, infectious viral particles that can drift around in the air for several hours. Outdoors, air currents can scatter and dilute the virus, making transmission less likely. You're more likely to inhale the virus indoors, with the windows closed, whether or not you have the air conditioning on.

If you must be indoors with anyone outside of your household, increase air circulation by keeping the windows open as much as possible.

Does vitamin D protect against COVID-19?

There is some evidence to suggest that vitamin D might help protect against becoming infected with, and developing serious symptoms of, COVID-19. We know, for example, that people with low vitamin D levels may be more susceptible to upper respiratory tract infections. One meta-analysis found that people who took vitamin D supplements, particularly those who had low vitamin D levels, were less likely to develop acute respiratory tract infections than those who didn't.

Vitamin D may protect against COVID-19 in two ways. First, it may help boost our bodies' natural defense against viruses and bacteria. Second, it may help prevent an exaggerated inflammatory response, which has been shown to contribute to severe illness in some people with COVID-19.

Our bodies make vitamin D when exposed to sunshine. Five to 10 minutes of sun exposure on some or most days of the week to the arms, legs, or back without sunscreen will enable you to make enough of the vitamin. Good food sources of vitamin D include fatty fish (such as tuna, mackerel, and salmon), foods fortified with vitamin D (such as dairy products, soy milk, and cereals), cheese, and egg yolks.

The recommended dietary dose of vitamin D is 600 IU each day for adults 70 and younger and 800 IU each day for adults over 70. A daily supplement containing 1,000 to 2,000 IU of vitamin D is likely safe for most people. For adults, the risk of harmful effects increases above 4,000 IU per day.

Is it safe to travel by airplane?

Stay current on travel advisories from regulatory agencies.

Anyone who is sick or has tested positive for COVID-19 should not fly if at all possible.

As a general rule, travel can increase your chance of spreading and getting COVID-19.

Is there a vaccine available?

On December 11, 2020, the FDA granted emergency use authorization (EUA) to an mRNA COVID-19 vaccine developed by Pfizer and BioNTech. This was the first COVID-19 vaccine authorized for use in the US. One week later, the FDA granted EUA to an mRNA COVID-19 vaccine developed by Moderna.

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

Early evidence suggests that the Pfizer/BioNTech mRNA vaccine is effective against the new coronavirus variants. Testing of the Moderna mRNA vaccine against the new variants is underway.

The two new variants are more contagious than previous SARS-CoV-2 variants, but they do not appear to be deadlier. One, called B.1.1.7, was first detected in the United Kingdom. The other, called B.1.351, originated in South Africa. Both variants have now been detected in countries around the globe.

Both 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. Interestingly, both 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.

To test the effectiveness of the Pfizer/BioNTech mRNA vaccine against the new variants, researchers created a virus with the N501Y mutation. They then took blood samples from 20 participants enrolled in their vaccine trial and exposed the samples to the mutated virus. The participants' blood contained human cells as well as antibodies made in response to the Pfizer/BioNTech vaccine. They found that the antibodies blocked the mutated virus from infecting human cells as effectively as it blocked the virus without the mutation.

More study is needed to see if these results hold up in real world conditions. It's also not known how long this protection may last, and whether the vaccines will work against other mutations found in these variants.

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

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

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

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

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

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

This vaccine requires two doses, spaced four weeks apart. We do not yet know how long immunity from this vaccine will last.

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

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

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

On December 11, 2020, the FDA granted emergency use authorization (EUA) to an mRNA COVID-19 vaccine developed by Pfizer and BioNTech. The vaccine is approved for use in people 16 years and older. This is the first COVID-19 vaccine authorized for use in the US.

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

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

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

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

This vaccine requires two doses, spaced three weeks apart. Although the vaccine appears to provide reasonable protection after the first dose, it provides stronger protection after two doses. We do not yet know how long immunity from this vaccine will last.

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

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

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

Even people who have already gotten sick with COVID-19 may benefit from the vaccine, according to the CDC.

Here's what we know. COVID-19 can lead to serious illness and long-term complications, even in younger people and those without underlying medical conditions. We also know that it's possible for someone who has already had COVID-19 to be re-infected, though this is not common.

On the other hand, we don't know how long natural immunity to COVID-19 — the protection that results from having been sick — lasts. It's not clear if the strength or duration of natural immunity varies based on the severity of the initial illness. We also don't know how long immunity conferred by vaccines lasts.

During the initial deployment of the vaccine to front line workers and people in long-term care facilities like nursing homes, people eligible for the vaccine will get it regardless of whether or not they were previously infected. They are not being tested for antibodies prior to vaccination.

As scientists learn more about natural immunity after COVID illness, vaccination criteria based on the presence of antibodies may play a role in the future.

Will the COVID vaccine prevent me from infecting others?

The answer is, we don't know.

Clinical trials of the Pfizer/BioNTech and Moderna vaccines found that both do a good job preventing symptomatic COVID-19 disease, including severe COVID-19. However, the trials did not measure whether a person who is vaccinated is less likely to spread the virus to someone else.

It's possible that the vaccines protect against COVID-19 disease by preventing a person from becoming infected in the first place. However, it's also possible that the vaccine protects a person from COVID-19 illness, but does not prevent a person from becoming infected. In other words, a vaccinated person may have replicating virus in their nose and throat even if they are protected from becoming sick.

But does that mean that you have enough virus in your nose and throat to infect someone else? Not necessarily. It's possible that the immune response triggered by the vaccine, which protects you from becoming sick, also reduces the amount of virus in your nose and throat to a point where you are unlikely to spread it to someone else. But we need more research to know for sure.

The bottom line? If you're among the first groups of people to get vaccinated, it's best to continue wearing masks and maintaining physical distance in order to protect others who haven't yet gotten the vaccine.

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

The vaccine will protect you from getting sick, but it may not prevent you from infecting others. That's why, at least for now, you should continue wearing a mask and physically distancing from others, even after you get your shot.

Clinical trials of the Pfizer/BioNTech and Moderna vaccines found that both do a good job preventing symptomatic COVID-19 disease, including severe COVID-19. However, the trials did not measure whether a person who is vaccinated is less likely to spread the virus to someone else.

It's possible that the vaccines protect against COVID-19 disease by preventing a person from becoming infected. However, it's also possible that the vaccine protects a person from getting sick but does not prevent the virus from replicating in that person's nose and throat.

Does that mean there's enough virus in your nose and throat to infect someone else? Not necessarily. But we need more research to know for sure.

Where does that leave us? If you're among the first groups of people to get vaccinated, it's best to continue wearing masks and maintaining physical distance in order to protect others who haven't yet gotten the vaccine.

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

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

Approval of a vaccine requires completion of the following steps:

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

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

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

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

Could an mRNA vaccine change my DNA?

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

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

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

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

The CDC, American College of Obstetrics and Gynecology (ACOG), and Society for Maternal-Fetal Medicine agree that the new mRNA COVID-19 vaccines (Pfizer/BioNTech and Moderna) should be offered to pregnant and breastfeeding individuals. But there is a lot we still do not know about the safety of the vaccines in these populations, and your own doctor is in the best position to advise you based on your personal health risks and preferences.

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

mRNA vaccine trials did not deliberately include pregnant individuals, so our knowledge regarding the safety of mRNA COVID-19 vaccines during pregnancy is limited. In animal studies, mRNA vaccines did not affect fertility or cause any problems with pregnancy. mRNA vaccines do not contain any virus, so they cannot cause COVID-19 in a woman or her baby. And our bodies quickly break down and eliminate mRNA particles used in the vaccine, so they are unlikely to reach or cross the placenta. On the other hand, the immunity that a woman generates from vaccination can cross the placenta, and may help to keep the baby safe after birth.

Experts also believe it is most likely safe to get an mRNA COVID-19 vaccine if you're breastfeeding, although breastfeeding people were not included in the vaccine trials. When a person gets vaccinated while breastfeeding, their immune system develops antibodies that protect against COVID-19. These antibodies can be passed through breast milk to the baby and are likely to help protect against infection.

What are adenovirus vaccines? What do we know about adenovirus vaccines that are being developed for COVID-19?

Adenoviruses can cause a variety of illnesses, including the common cold. They are being used in two leading COVID-19 vaccine candidates as capsules (the scientific term is vectors) to deliver the coronavirus spike protein into the body. The spike protein prompts the immune system to produce antibodies against it, preparing the body to attack the SARS-CoV-2 virus if it later infects the body.

In December 2020, AstraZeneca published promising results of an adenovirus-based vaccine that it developed with researchers at the University of Oxford. The study was published in the journal The Lancet.

The Lancet analysis of vaccine efficacy was based on 11,636 adult study participants. Of these, 4,440 participants received a full dose of the coronavirus vaccine, followed four weeks later by another full dose. Nearly 1,400 participants received a half dose of the coronavirus vaccine, followed four weeks later by a full dose. The control group received a meningitis vaccine, followed by a second meningitis vaccine or a placebo (a salt water shot). There were 131 documented cases of COVID-19, all of which occurred at least two weeks after the second shot.

The coronavirus vaccine reduced the risk of COVID-19 by an average of 70.4%, compared to the control group. Surprisingly, the half dose/full dose vaccine combination was more effective, reducing risk of COVID-19 by 90%. The full dose combination reduced risk by 62%. None of the participants who received the coronavirus vaccine developed severe COVID-19 or had to be hospitalized. There was also a reduction in asymptomatic cases.

Most study participants were between the ages of 18 and 55, and white. In addition, the study participants were healthy or had stable underlying medical conditions. More data is needed to understand how effective this vaccine is in people older than 55, people of color, and people with underlying medical conditions. This vaccine is in clinical trials around the world, including the US. But this analysis was based on data from the United Kingdom and Brazil.

The adenovirus used in the AstraZeneca/University of Oxford vaccine is a weakened, harmless form of a chimpanzee common-cold adenovirus. This vaccine can be safely refrigerated for several months.

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

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

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

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

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

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

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

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

Two other vaccine approaches are also showing promise.

  • A hybrid vaccine created by AstraZeneca and the University of Oxford uses a modified, harmless form of a chimpanzee common-cold adenovirus to deliver the coronavirus spike protein into the body. The spike protein prompts the immune system to produce antibodies against it, preparing the body to attack the SARS-CoV-2 virus if it later infects the body. The company published promising phase 3 trial results in The Lancet in December 2020.
  • Another hybrid vaccine uses a human common-cold adenovirus to deliver the coronavirus spike protein into the body. That platform was developed by Harvard Medical School scientists in collaboration with Johnson and Johnson. This vaccine is in phase 3 trials.

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

Can a person who has been infected with coronavirus get infected again?

Natural immunity to COVID-19 is the protection that results from having been sick. But we don't know how long natural immunity lasts, or how strong it is. So can we count on natural immunity to protect us from reinfection? If so, for how long? Unfortunately, we don't know the answer to either of those questions.

There have been some confirmed cases of reinfection with COVID-19. In other words, a person got sick with COVID-19, recovered, and then became infected again. This is rare, but it can happen.

The CDC states that people who have gotten sick with COVID-19 may still benefit from getting vaccinated. For now, people are eligible to get the COVID-19 vaccine whether or not they were previously infected. Vaccination criteria may change in the future as scientists learn more about natural immunity after COVID illness.

It's also worth noting that someone who has been reinfected — even someone with no symptoms — has the potential to spread the virus to others. That means that everyone, even those who have recovered from coronavirus infection and those who have been vaccinated, should continue to wear masks, practice physical distancing, and avoid crowds.

What is herd immunity, and could it play a role in stopping the spread of COVID-19?

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.

Experts initially estimated that somewhere between 60% and 70% of the population needs to be immune in order to achieve herd immunity. More recently, they have raised that estimate to near 90%. (As of now, we are nowhere close to the numbers needed to achieve herd immunity.)

Achieving herd immunity through natural infection means many people would become ill and many would die. These risks may fall as we develop effective treatments. However, we still don't know how long people who recover from COVID-19 will remain immune to reinfection.

Ideally, we will achieve herd immunity as more people around the world receive vaccines that will confer lasting immunity.

Will a pneumococcal vaccine help protect me against coronavirus?

Vaccines against pneumonia, such as pneumococcal vaccine and Hemophilus influenza type B (Hib) vaccine, only help protect people from these specific bacterial infections. They do not protect against any coronavirus pneumonia, including pneumonia that may be part of COVID-19. However, even though these vaccines do not specifically protect against the coronavirus that causes COVID-19, they are highly recommended to protect against other respiratory illnesses.

Can my pet infect me with the virus that causes COVID-19?

At present, it is considered unlikely that pets such as dogs or cats can spread the COVID-19 virus to humans. However, pets can spread other infections that cause illness, including E. coli and Salmonella, so wash your hands thoroughly with soap and water after interacting with pets.

Can people infect pets with the COVID-19 virus?

The virus that causes COVID-19 does appear to spread from people to pets, according to the FDA. Research has found that cats and ferrets are more likely to become infected than dogs.

If you have a pet, do the following to reduce their risk of infection:

  • Avoid letting pets interact with people or animals that do not live in your household.
  • Keep cats indoors when possible to prevent them from interacting with other animals or people.
  • Walk dogs on a leash maintaining at least six feet from other people and animals.
  • Avoid dog parks or public places where a large number of people and dogs gather.

If you become sick with COVID-19, restrict contact with your pets, just like you would around other people. This means you should forgo petting, snuggling, being kissed or licked, and sharing food or bedding with your pet until you are feeling better. When possible, have another member of your household care for your pets while you are sick. If you must care for your pet while you are sick, wash your hands before and after you interact with your pets and wear a face mask.

What can I do to keep my immune system strong?

Your immune system is your body's defense system. When a harmful invader — like a cold or flu virus, or the coronavirus that causes COVID-19 — gets into your body, your immune system mounts an attack. Known as an immune response, this attack is a sequence of events that involves various cells and unfolds over time.

Following general health guidelines is the best step you can take toward keeping your immune system strong and healthy. Every part of your body, including your immune system, functions better when protected from environmental assaults and bolstered by healthy-living strategies such as these:

  • Don't smoke or vape.
  • Eat a diet high in fruits, vegetables, and whole grains.
  • Take a multivitamin if you suspect that you may not be getting all the nutrients you need through your diet.
  • Exercise regularly.
  • Maintain a healthy weight.
  • Control your stress level.
  • Control your blood pressure.
  • If you drink alcohol, drink only in moderation (no more than one to two drinks a day for men, no more than one a day for women).
  • Get enough sleep.
  • Take steps to avoid infection, such as washing your hands frequently and trying not to touch your hands to your face, since harmful germs can enter through your eyes, nose, and mouth.

Should I go to the doctor or dentist for nonurgent appointments?

Many medical and dental practices have instituted comprehensive safety measures to help protect you, the doctor and office staff, and other patients. If you feel anxious about visiting in person, call the practice.

Many doctor's offices are increasingly providing telehealth services. This may mean appointments by phone call, or virtual visits using a video chat service. Ask to schedule a telehealth appointment with your doctor for a new or ongoing nonurgent matter. If, after speaking to you, your doctor would like to see you in person, he or she will let you know.

What if your appointments are not urgent but also don't fall into the low-risk category? For example, if you have been advised to have periodic scans after cancer remission, if your doctor sees you regularly to monitor for a condition for which you're at increased risk, or if your treatment varies based on your most recent test results? In these and similar cases, call your doctor for advice.

Should I postpone my elective surgery?

The availability of elective surgeries and procedures throughout the United States is very fluid, and may reflect the number of cases and infection rate in a given area. If COVID-19 cases are rising in your area, it's quite possible that you already have been canceled or rescheduled by the hospital or medical center in which you are scheduled to have the procedure. If not, then you should consider postponing any procedure that can wait.

That being said, keep in mind that "elective" is a relative term. For instance, you may not have needed immediate surgery for sciatica caused by a herniated disc. But the pain may be so severe that you would not be able to endure postponing the surgery for weeks or perhaps months. In that case, you and your doctor should make a shared decision about proceeding.

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For more information on coronavirus and COVID-19, see the Harvard Health Publishing Coronavirus Resource Center.

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