Preventing the spread of the coronavirus

Social distancing, hand washing, and other preventive measures

Published: March, 2020

You've gotten the basics down: you're washing your hands regularly and keeping your distance from friends and family. But you likely still have questions. Are you washing your hands often enough? How exactly will social distancing help? What's okay to do while social distancing? And how can you strategically stock your pantry and medicine cabinet in order to minimize trips to the grocery store and pharmacy?

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:

  • Avoid close contact with people who are sick.
  • Avoid 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 every day. High touch surfaces include counters, tabletops, doorknobs, bathroom fixtures, toilets, phones, keyboards, tablets, and bedside tables. A list of products suitable for use against COVID-19 is available here. This list has been pre-approved by the US Environmental Protection Agency (EPA) for use during the COVID-19 outbreak.
  • 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 is thought to spread 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 cover their nose and mouth when they go out in public.

Coronavirus can also spread from contact with infected surfaces or objects. For example, a person can get COVID-19 by touching a surface or object that has the virus on it and then touching their own mouth, nose, or possibly their eyes.

The virus may be shed in saliva, semen, and feces; whether it is shed in vaginal fluids isn't known. Kissing can transmit the virus. Transmission of the virus through feces, or during vaginal or anal intercourse or oral sex, appears to be extremely unlikely at this time.

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 social distancing and why is it important?

The COVID-19 virus primarily spreads when one person breathes in droplets that are produced when an infected person coughs or sneezes. In addition, any infected person, with or without symptoms, could spread the virus by touching a surface. The coronavirus could remain on that surface and someone else could touch it and then touch their mouth, nose or eyes. That's why it's so important to try to avoid touching public surfaces or at least try to wipe them with a disinfectant.

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 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 social 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. Highly realistic projections show that unless we begin extreme social distancing now — every day matters — our hospitals and other healthcare facilities will not be able to handle the likely influx of patients.

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.

How many COVID-19 infections may have been prevented because of policies designed to slow the spread of the virus?

Large-scale emergency health measures prevented more than 500 million COVID-19 infections in six countries between January and early April 2020, according to a peer-reviewed article published in the journal Nature. The study authors looked at the impact of policies designed to slow the spread of the virus in six countries: the United States, China, South Korea, Italy, Iran, and France. They compared the growth rate of COVID-19 infections in each country before and after the policies — such as travel restrictions; business, school, and restaurant closures; and social distancing — were enacted.

These findings reinforce the importance of continuing to exercise caution and practice health-promoting behaviors as restrictions begin to ease.

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.

Should I keep extra food at home? What kind?

Consider keeping a two-week to 30-day supply of nonperishable food at home. These items can also come in handy in other types of emergencies, such as power outages or snowstorms.

  • canned meats, fruits, vegetables, and soups
  • frozen fruits, vegetables, and meat
  • protein or fruit bars
  • dry cereal, oatmeal, or granola
  • peanut butter or nuts
  • pasta, bread, rice, and other grains
  • canned beans
  • chicken broth, canned tomatoes, jarred pasta sauce
  • oil for cooking
  • flour, sugar
  • crackers
  • coffee, tea, shelf-stable milk, canned juices
  • bottled water
  • canned or jarred baby food and formula
  • pet food
  • household supplies like laundry detergent, dish soap, and household cleaner.

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 shouldn't go to 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). Avoid public playground structures, which aren't cleaned regularly and can spread the virus.

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, 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 everyone in the US wear nonsurgical masks when going out in public.

Coronavirus primarily spreads when someone breathes in droplets containing virus that are produced when an infected person coughs or sneezes or when a person touches a contaminated surface and then touches their eyes, nose, or mouth. But people who are infected but do not have symptoms, or have not yet developed symptoms, can also infect others. That's where masks come in.

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. Another person can breathe in these aerosols and become infected with the virus. A mask can help prevent that spread. An article published in NEJM in March reported that aerosolized coronavirus could remain in the air for up to three hours.

What kind of mask should you wear? Because of the short supply, people without symptoms or without exposure to someone known to be infected with the coronavirus can wear a cloth face covering over their nose and mouth. They do help prevent others from becoming infected if you happen to be carrying the virus unknowingly.

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

Some parts of the US also have inadequate supplies of surgical masks. If you have a surgical mask, you may need to reuse it at this time. But never share your mask.

Surgical masks are preferred if you are caring for someone who has COVID-19 or you have any respiratory symptoms (even mild symptoms) and must go out in public.

Masks are more effective when they are tight-fitting and cover your entire nose and mouth. They can help discourage you from touching your face (be sure you're not touching your face more often to adjust the mask). Masks are meant to be used in addition to, not instead of, physical distancing.

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.

How much difference will masks and physical distancing make as states begin to reopen?

Although we don't know exactly how much masks and physical distancing help, we do know that these measures are needed to open the economy in the safest way possible and are our best chance of keeping it open. But in order to work, everyone must comply.

Coronavirus spreads when someone breathes in virus that an infected person emits through coughs or sneezes, or when they talk or breathe, or when a person touches a contaminated surface and then touches their eyes, nose, or mouth. Physical distancing of at least six feet and wearing a tight-fitting cloth mask that covers your nose and mouth can help prevent spread. (Medical-grade N95 masks are more effective than cloth masks, but are in short supply and should be reserved for healthcare workers.) Continue to wash your hands frequently as well.

Researchers at Columbia University recently affirmed the importance of these health-promoting behaviors. They reported that 36,000 fewer people would have died in the coronavirus outbreak if physical distancing measures has started one week earlier; if physical distancing had begun two weeks earlier, 54,000 fewer people may have died.

Could wearing masks prevent COVID deaths?

According to a new 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.

As I start seeing more friends and family, does it matter whether we meet indoors or outdoors?

As you gradually expand your social circle, 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. A recent study found 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. This is a rapidly changing situation.

Anyone who has a fever and respiratory symptoms should not fly if at all possible. Even if a person has symptoms that feel like just a cold, he or she should wear a mask on an airplane.

Is there a vaccine available?

No vaccine is available, although more than 70 vaccine candidates are in various phases of clinical trials.

What needs to happen before we have a vaccine for COVID-19?

A successful COVID-19 vaccine will safely and effectively stimulate the body's immune system to create antibodies that protect against the COVID-19 coronavirus. More than 100 COVID-19 vaccines are currently in various stages of development and testing, and more than one may eventually be approved.

Anti-virus vaccine development traditionally requires 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.

Some researchers are combining trial phases in an effort to speed up the COVID-19 vaccine development process. In addition, the US government has created Operation WARP Speed, which will identify and support mass production of the most promising vaccines before they have been approved, so that they will be available quickly if they are ultimately found to be safe and effective. It's likely that a COVID-19 vaccine will be available before we know how long that vaccine's protective effect will last.

What are mRNA vaccines? What do we know about mRNA vaccines that are being developed for 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 produces antibodies against it. If the antibodies later encounter the actual virus, they are ready to recognize and destroy it before it causes illness.

In the past couple of weeks, two companies have released promising data about their mRNA vaccines. Results for both vaccines were reported in company press releases, not in peer reviewed scientific journals.

One of the mRNA vaccines was developed by Pfizer and BioNTech. Their phase 3 clinical trial found that their vaccine reduced the risk of infection by 95%. The trial enrolled nearly 44,000 adults. Of these, 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. This suggests that the vaccine reduces risk of both mild and severe COVID. The vaccine was consistently effective across age, race, and ethnicity. Of the US study participants, 30% were people of color and 45% were age 56 to 85.

The other mRNA vaccine, developed by Moderna, released an interim analysis of its phase 3 trial, announcing that its vaccine was 94.5% effective. This study enrolled 30,000 adults; half received the vaccine, half received a saltwater placebo shot. There were 95 infections among the study participants. Of these, 90 were in the placebo group and 5 were in the vaccine group. All 11 severe COVID cases occurred in the placebo group. This vaccine also appears to reduce risk of mild and severe illness. And it was effective in older people, people with medical conditions that put them at high risk for severe illness, and in racial and ethnic minorities, which made up 37% of the study participants. The study enrolled more than 7,000 participants older than 65, and more than 5,000 people under 65 who were at high risk for severe illness.

Both vaccines had a good safety record. Side effects included fatigue, headache, and muscle pain.

These results are promising, but there are still questions left to be answered. For example, we do not yet know how long immunity from these vaccines will last. Both of these vaccines require two doses (three weeks between shots for the Pfizer vaccine and four weeks between shots for the Moderna vaccine), and we don't know how effective the vaccine is in people who only get one dose. There is also the question of storage. mRNA vaccines must be stored at very cold temperatures, and improperly stored vaccines can become inactive.

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 a press release, AstraZeneca announced promising preliminary results of an adenovirus-based vaccine that it developed with researchers at the University of Oxford.

The preliminary analysis was based on more than 23,000 adult study participants enrolled in a phase 3 clinical trial. Of these, nearly 9,000 participants received a full dose of the coronavirus vaccine, followed four weeks later by another full dose. Nearly 3,000 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 saltwater 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.

All study participants were healthy or had stable 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?

Healthcare workers and residents and staff of long-term care facilities will get the first COVID-19 vaccines once the vaccines are granted Emergency Use Authorization (EUA).

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.

Both Pfizer/BioNTech and Moderna have applied to the FDA for EUA of their vaccines. Pfizer's vaccine is expected to receive EUA in mid-December, and Moderna's vaccine soon after. Both of these vaccines require two doses spaced a few weeks apart. The companies estimate that they will have enough to vaccinate about 20 million people by the end of December, with vaccine production continuing to ramp up in early 2021. Other vaccines, including one by AstraZeneca, are also on the horizon.

The next priority groups for vaccination are expected to include essential workers, adults with underlying medical conditions that increase risk for severe COVID-19, and adults over age 65.

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.

Are there any 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.

The following three vaccine approaches are among the first to be tested clinically in the United States.

  • Two vaccines, one created by Moderna and the other created by Pfizer and BioNTech, 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 vaccine then triggers the immune system to make antibodies against the actual spike protein if a person is exposed to the virus. Both companies announced promising phase 3 trial results in company press releases in November 2020.
  • 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 announced promising preliminary phase 3 trial results in a press release in November 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.

In animal models, all four vaccines provide protective immunity against SARS-CoV-2, the COVID-19 virus. Ongoing trials will help establish their effectiveness in real-world circumstances and potential side effects. A central question will be how long protection might last.

Is a person who has been infected with coronavirus protected from becoming infected again?

Most people who are infected with the COVID-19 virus produce antibodies, which are proteins that make it harder for the virus to infect cells. But antibodies are only one part of the body's immune response. T cells, for example, can destroy cells that are already infected. And memory B cells can quickly generate a strong antibody response to a virus the body has encountered before.

Antibody levels typically fall once the immediate threat of infection declines. Several new studies have found that COVID-19 antibody levels decline, but then stabilize and remain in the blood even two to three months after infection. Other recent studies found COVID-specific B cells and T cells in the blood, months after people recovered. This all suggests that the immune system would be ready to react quickly and strongly if re-exposed to the COVID-19 virus.

The laboratory evidence is promising. But the real evidence will come as more and more people who have recovered (rather than cells in a lab) are re-exposed to the virus.

Researchers in Hong Kong recently reported the first confirmed case of reinfection with the COVID-19 virus. The man was first infected in March, then declared to be recovered after two negative coronavirus tests. Four and a half months later, he tested positive for coronavirus on a saliva screening test administered at the Hong Kong airport upon his return from a trip to Europe. Scientists sequenced viral genetic material from the man's two infections and found significant enough differences to suggest two separate infections.

The man had experienced mild symptoms during his first infection and had no symptoms the second time he was infected. It's possible that while his immune system could not protect him against reinfection, it did help keep him from getting sicker during the second infection.

It's far too early to make any generalizations. We cannot know from one case of reinfection (among tens of millions of people who have been infected) how strong an average immune response will be, or how long that immune response will last.

It's also worth noting that someone who has been re-infected — 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, should continue to wear masks and practice physical distancing.

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.

Based on what we know about the contagiousness of the COVID-19 virus, experts estimate that somewhere between 60% and 70% of the population needs to be immune in order to achieve herd immunity. That's close to 200 million people in the United States, and nearly five billion people worldwide. (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 through a safe vaccine (or vaccines) that will confer lasting immunity.

Will a pneumococcal vaccine help protect me against coronavirus?

Vaccines against pneumonia, such as pneumococcal vaccine and Haemophilus 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.

My husband and I are in our 70s. I'm otherwise healthy. My husband is doing well but does have heart disease and diabetes. My grandkids' school has been closed for the next several weeks. We'd like to help out by watching our grandkids but don't know if that would be safe for us. Can you offer some guidance?

People who are older and older people with chronic medical conditions, especially cardiovascular disease, high blood pressure, diabetes, and lung disease are more likely to have severe disease or death from COVID-19, and should engage in strict social distancing without delay. This is also the case for people or who are immunocompromised because of a condition or treatment that weakens their immune response.

The decision to provide on-site help with your children and grandchildren is a difficult one. If there is an alternative to support their needs without being there, that would be safest.

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.

At present, it is considered unlikely that pets 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 your animal companions.

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 now have sufficient personal protective equipment and 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?

Doctors have restarted performing elective surgeries and procedures in many communities. But the situation in the United States is very fluid, with a rapid increase in cases in some states. For those states with rising COVID-19 cases, 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 during this period of social distancing, 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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