Questions About COVID-19 Vaccination

Wondering which shot to get, when, and why? Find answers to those questions and more.

Which vaccines are being used in the U.S.?

So far three vaccines(link opens in new window), commonly referred to by the names of the companies that make them, are available in the United States:

How do the vaccines work?

The Pfizer and Moderna vaccines are a type called a messenger RNA vaccine. Johnson & Johnson's is a viral vector vaccine. Both types of vaccines teach the body how to fight the virus that causes COVID-19 without you actually having to get the illness. None of the vaccines causes the illness — there is no live virus in any of the vaccines. Research shows the vaccines can protect you from getting COVID-19, although they are not 100% effective, so some fully vaccinated people could still get infected. But more importantly, vaccination has been shown to greatly reduce the risk of getting seriously ill if you do get COVID-19.

How long will the vaccine protect me?

Scientists are still studying that question, although recent research has indicated that the level of protection wanes over time — leading public health authorities to recommend that everyone age 12 and older who has been vaccinated get a booster shot. The U.S. Centers for Disease Control and Prevention currently recommends only one booster shot.

I've heard some people should get third shots. Who are they?

The CDC has tailored guidance for people whose immune systems are compromised (see below). Everyone else age 18 and older who had the Pfizer-BioNTech or Moderna COVID-19 vaccine is advised to get a third, or "booster," shot of either of these vaccines at least five months after receiving their second vaccination in the two-shot series. Also, kids ages 12 to 17 are eligible for a Pfizer booster on that same five-month timetable after completing the initial Pfizer vaccination series.

If you received the Johnson & Johnson vaccine, a booster shot with the Pfizer or Moderna vaccine is advised two months after your original shot. The Johnson & Johnson vaccine can be considered for booster shots in some situations.

Boosters of each shot are the same formulation as given for initial vaccination, although the Moderna booster is delivered at half the original dose.

What should I do if my immune system is compromised?

Vaccination recommendations differ for people with substantially weakened immune systems, depending on which vaccine they originally received. Here's what the CDC says:

  • Anyone age 5 or older who is immunocompromised and has had two Pfizer shots should get an additional shot of the same vaccine as part of the primary vaccination series, 28 days after the second shot. Five months later, those 12 and older should get a booster, preferably with a Pfizer or Moderna shot.
  • If you're 18 or older and had the Moderna shot, you're advised to get an additional primary series shot of the same vaccine 28 days after your second shot. Five months later, you should get a Moderna or Pfizer booster.
  • If you received the Johnson & Johnson vaccine, no additional primary shot is currently advised. But a booster shot of any of the available vaccines is recommended at least two months after your original vaccination.

Conditions such as advanced HIV infection and some genetic disorders can compromise the immune system. So can treatments including stem cell transplantation, some anti-cancer medicines, high-dose corticosteroids and immune-suppressing drugs used after organ transplantation. Check with your health care team to see whether you should get a third shot. Also, continue other COVID-19 precautions including masking, distancing and frequent handwashing, and be sure people close to you are vaccinated whenever possible.

If you're getting additional vaccine doses, don't forget to take your original CDC COVID-19 Vaccination Record Card. As with the original vaccination series, additional shots are free to all people in the U.S.

What is “emergency use authorization”? How does it differ from FDA "approval"?

All the vaccines available in the U.S. have received an official designation called emergency use authorization from the Food and Drug Administration. During a public health emergency such as the COVID-19 pandemic, the FDA can allow the use of medical products such as the new vaccines that haven't yet received full agency approval if they are needed to fight serious or life-threatening disease and no acceptable alternatives are available. In evaluating the evidence for an emergency use authorization, the agency must decide that the known and potential benefits outweigh the known and potential risks of the medical product in question.

For full approval, the COVID-19 vaccines must clear the FDA's standard, more comprehensive review process for quality, safety and effectiveness, which builds on the extensive data previously submitted for the emergency use authorization. In August 2021, the Pfizer vaccine became the first fully approved COVID-19 vaccine for people ages 16 and older. It continues to be available under emergency use authorization for people ages 12 to 15 and for the third dose that is being given to people with moderately to severely weakened immune systems.

I heard there were concerns about the Johnson & Johnson COVID-19 vaccine. What are they?

In December, an expert committee of the CDC evaluated the effectiveness, safety and rare adverse events associated with the Johnson & Johnson Janssen vaccine. The committee recommended the two other available vaccines, the mRNA products from Pfizer and Moderna, as preferred choices over the Johnson & Johnson vaccine.

The J&J shot will continue to be available in the U.S. for people who can't or don't wish to receive the Pfizer or Moderna vaccine series. Any vaccine is better than being unvaccinated, the expert committee noted.

Here's the background behind that recommendation:

In July 2021, the U.S. Food and Drug Administration noted preliminary reports of people developing a neurological disorder called Guillain-Barré syndrome after receiving the Johnson & Johnson Janssen COVID-19 vaccine. In Guillain-Barré, the immune system attacks nerve calls, causing muscle weakness or sometimes paralysis. Most people fully recover from the condition.

Besides weakness, symptoms include difficulty with eye muscles and vision; trouble swallowing, speaking or chewing; "pins and needles" sensation in the hands and feet; and coordination problems.

The case reports associated with vaccination are extremely rare: As of late November 2021, among over 16.4 million patients receiving the Johnson & Johnson vaccine, there were about 268 preliminary reports of the illness. Cases have been reported typically about two weeks after vaccination and mostly in men, many 50 and older.

Also, in mid-April 2021 the FDA and the Centers for Disease Control and Prevention briefly paused the use of the Johnson & Johnson vaccine while they examined reports of an exceptionally rare but extreme complication called thrombosis-thrombocytopenia syndrome (TTS).

TTS involves clotting, or thrombosis, in the cerebral venous sinuses (where it can prevent blood from draining out of the brain and cause a stroke), or in other sites including the large blood vessels of the abdomen and veins in the legs. This clotting is counterintuitively combined with thrombocytopenia, or low levels of blood platelets, which help blood clot.

During the pause, FDA and CDC teams reached out to health care providers to make sure they were aware of this rare adverse event and could recognize it and safely manage it. The agencies also have provided updated information for patients and caregivers.

As of late November 2021, experts had confirmed 54 cases of TTS out of over 16.4 million doses given of the Johnson & Johnson vaccine. Six TTS fatalities were associated with the vaccine. Women 50 and younger should be aware of this risk, the CDC says, noting that other vaccines that have not been linked to this condition (the Pfizer and Moderna vaccines) are available. Risk is extremely low for women older than 50 and men of all ages, the agency says.

People who get the vaccine should seek medical care right away if they have any of these symptoms afterward:

  • Shortness of breath
  • Chest pain
  • Leg pain or swelling
  • Persistent abdominal or back pain
  • Severe or persistent headaches or blurred vision
  • Easy bruising or tiny blood spots under the skin beyond the site of the injection
  • Nausea and vomiting

These symptoms are different from the flu-like symptoms such as fever that are common after vaccination.

After their review in April 2021 of the TTS cases and other vaccine safety data, the agencies concluded that use of the Johnson & Johnson vaccine should resume. Officials continue to monitor safety data closely.

Are these conditions a problem with the other vaccines?

Neither of these conditions, TTS or Guillain-Barré, has been definitively associated with the Pfizer and Moderna vaccines, the only others being used in the U.S. — and over 500 million doses of those have been given so far. However, there have been reports of an inflammatory condition called myocarditis with these vaccines (see "side effects" question below). Research shows the benefits of COVID-19 vaccination outweigh the low risk of myocarditis after receiving the vaccine.

Are there serious side effects from the vaccine?

Very rarely, a severe, dangerous allergic reaction called anaphylaxis can occur shortly after your shot. Symptoms of anaphylaxis include a swollen throat, a rash that can itch, wheezing or other trouble breathing, and passing out. If you think you are having such a severe reaction after you leave your vaccination site, call 911 immediately.

Experts have estimated anaphylaxis has occurred in 2 to 5 cases per million vaccinated in the U.S. These reactions most commonly happen within 15 minutes of when the shot was given — that's why there's a short waiting period, with health care workers standing by if treatment is needed, after your vaccination.

You could also have a less severe allergic reaction within four hours of your shot, with symptoms such as hives, swelling and wheezing. If you have any of these allergic reactions, the CDC recommends you not get a second shot of that type of vaccine.

In addition, public health experts are monitoring vaccine safety and side effect data for cases of rare heart-related complications, myocarditis or pericarditis, after receiving the Pfizer or Moderna vaccines. (Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the heart's outer lining.)

As of late November 2021, officials had confirmed 1,071 reports nationwide of myocarditis or pericarditis following vaccination. Cases were more common in adolescent males and young adults. A recent study shows that most people under age 21 who developed myocarditis thought to be linked to COVID-19 vaccination had mild symptoms that improved quickly.
The American Heart Association/American Stroke Association urges all U.S. adults and children 5 and older to receive COVID-19 vaccination as soon as possible to protect against severe illness from the coronavirus. Learn more here.

Should I get vaccinated if I'm pregnant — or planning to be?

Yes. Pregnant or recently pregnant women face an elevated risk of death or severe illness from COVID-19, including illness requiring hospitalization, intensive care, or a ventilator or other special equipment to breathe, according to the CDC. Also, risk of pregnancy complications with COVID-19 is higher. Women who are breastfeeding or might become pregnant in the future also are urged to get vaccinated. There is no evidence that any vaccines, including those protecting against COVID-19, cause fertility problems for women or men, the CDC says.

How should I get ready for my vaccination appointment? What should I expect?

If you have a chance to fill out paperwork online before the visit, you’ll save time at your appointment. Plan for only light activity for a day or so after you receive your vaccine, in case you have any effects from the shot, like soreness or feeling a little ill.

Check with your doctor or vaccine provider about whether any medicines you are taking might be a problem. The CDC advises against taking antihistamines or over-the-counter painkillers such as acetaminophen, aspirin or ibuprofen solely as a measure to prevent vaccine side effects.

Bring your photo ID and your insurance card (if applicable). If this is your second shot, bring the CDC COVID-19 Vaccination Record Card that you received at your first visit.

Be sure that you and any unvaccinated person who accompanies you is masked. (The CDC offers tips to improve the effectiveness of masking with a better fit and better filtration.) Continue to practice social distancing while in line for your vaccination.

The actual shot, given in the upper arm muscle, will only take a few seconds. Afterward, you will be asked to wait 15 to 30 minutes to ensure that a risky but very rare and treatable allergic reaction, called anaphylaxis, does not occur. Health care workers onsite are ready to treat anyone who has this reaction.

If I'm getting fully vaccinated for the first time, when should I get the second COVID-19 shot?

When you were first vaccinated, your provider should have advised you about whether you'll need a second shot and how to schedule it. If you're unsure, check the CDC COVID-19 Vaccination Record Card you received at your first visit. If you were given the Pfizer vaccine, you should have a second shot 21 days after the first. If you got the Moderna vaccine, you'll need a second shot 28 days after the first. The back of your card might list when your second shot is due, although providers are often scheduling appointments separately online or by phone.

If you had the Johnson & Johnson Janssen vaccine, a booster shot is recommended two or more months later.

Can I get my COVID-19 vaccination at the same time as another vaccine?

Yes. The CDC says a COVID-19 vaccine and other immunizations, such as a flu shot, can be given together, or without any specific time interval in between.

Help! Where can I get the vaccine?

As the vaccine supply has increased, finding a vaccine has become much easier than it was early on. Check with your local pharmacies — many are part of a federal program to increase vaccine access. You can also use the VaccineFinder tool to search for vaccine providers in your area, or check the website of your state or territorial health department. The vaccine might also be available at your primary care provider's office; if not, staff there can guide you.

What are the most common side effects of vaccination?

Many people have no side effects from COVID-19 vaccination. Many others experience minor effects for a few days, including:

  • Pain, redness or swelling at the injection site
  • Fatigue
  • Headache or muscle pain
  • Chills or fever
  • Nausea

Side effects are more common after the second shot in the two-shot series. Call your health care team if these effects last more than a few days, are worrisome or if the redness and swelling in your arm worsens after 24 hours. For minor issues, check out this advice from the CDC.

You can report side effects directly to the CDC through a new, smartphone-based tool called v-safe. (You will need the card you were given when you were vaccinated, which records the date and type of vaccination you received as well as the location).

Are there serious side effects from the vaccine?

Very rarely, a severe, dangerous allergic reaction called anaphylaxis can occur shortly after your shot. Symptoms of anaphylaxis include a swollen throat, a rash that can itch, wheezing or other trouble breathing, and passing out. If you think you are having such a severe reaction after you leave your vaccination site, call 911 immediately.

Early research has found that among almost 1.9 million doses of the Pfizer vaccine administered in December 2020, just 21 cases of anaphylaxis were reported. Over 70% of these happened within 15 minutes of when the shot was given — that's why there's a short waiting period, with health care workers standing by, after your vaccination.

You could also have a less severe allergic reaction within four hours of your shot, with symptoms such as hives, swelling and wheezing. If you have either of these allergic reactions, the CDC recommends you not get a second shot of that type of vaccine.

In addition, public health experts are monitoring vaccine safety and side effect data for cases of rare heart-related complications, myocarditis or pericarditis, after receiving the Pfizer or Moderna vaccines. (Myocarditis is inflammation of the heart muscle, while pericarditis is inflammation of the heart's outer lining.)

As of mid-October 2021, officials had confirmed 945 reports nationwide of myocarditis or pericarditis out of nearly 400 million doses given of the two vaccines. Cases were more common in adolescent males and young adults.

The American Heart Association/American Stroke Association urges all U.S. adults and children 12 and older to receive a COVID-19 vaccine as soon as possible, noting that the benefits of vaccination outweigh the rare, possible risk of heart-related complications. Learn more here.

Can I catch COVID-19 from the vaccine?

No. None of the vaccines available in the U.S. contains the live virus. But because the vaccines teach the body to recognize and fight the virus that causes COVID-19, you might have side effects like fever, which can occur when the immune system is working hard — building your immunity to the virus.

Also, it's possible you could already have COVID-19 but not be feeling symptoms before vaccination — or could catch the disease right after your shot, before your immune system can protect you. So be sure to keep following COVID-19 precautions including masking and physical distancing.

Could vaccination cause me to test positive for COVID-19?

You won't test positive on viral tests, which are used to see whether you are currently infected. A viral test is recommended if:

  • you have COVID-19 symptoms
  • you've had close contact with someone who has confirmed COVID-19
  • you've been in situations where you can't socially distance, such as travel, large gatherings or crowded indoor settings
  • you've been referred for testing by a health care or public health professional

You might test positive on some antibody tests, which look for signs of an immune system response to a previous infection. Experts are still studying this.

Will the vaccine keep me from spreading COVID-19 to others?

Research suggests the vaccines can help limit the spread of COVID-19 (in addition to preventing severe illness or death in you).

When am I considered “fully vaccinated”?

You're fully vaccinated, or fully protected, two weeks after the last shot required for the type of vaccine you receive. That means if you received:

  • the Pfizer or Moderna vaccines, each of which requires two doses, you are fully protected two weeks after the second dose.
  • the one and only shot required for the Johnson & Johnson Janssen vaccine, you are fully protected two weeks later.

If you haven't received a vaccine, or are between doses in the two-shot series, continue taking all COVID-19 precautions(link opens in new window) when you’re out in public. This includes wearing a mask, avoiding crowds and poorly ventilated spaces, staying at least 6 feet away from others and monitoring your health.

Once I'm fully protected, can I go back to living my life the way I did before the pandemic?

If your area has "substantial" or "high" levels of COVID-19 transmission in the community (check this COVID Data Tracker), the CDC advises fully vaccinated people, along with unvaccinated people, to wear a mask in public indoor settings (more guidance is here).

With the rise of the coronavirus's Delta variant, research has shown that fully vaccinated people can contract COVID-19 — and vaccinated people can be contagious, the CDC said recently in updating its mask guidelines for vaccinated people.

In areas with low levels of coronavirus transmission, fully vaccinated people generally don't need masks, indoors or out, with a few exceptions — for instance, if you have a weakened immune system. You also might wear a mask in public if someone in your household is immunocompromised, is not fully vaccinated or has a medical condition that increases risk for severe disease.

Masks are still required for fully vaccinated people in some circumstances, such as:

  • Where state or local law mandates them
  • In health care settings, businesses and workplaces that require masking
  • When using public transportation and in related facilities such as airports and bus depots

Traveling is more complicated. The CDC provides guidance for domestic and international travel, and has a travel planner that lets you learn about specific U.S. destinations.

Even after vaccination, you should still be alert to any COVID-19 symptoms you might have. If symptoms arise, get tested, stay home, and avoid others.

What's a “vaccine passport”?

National governments and other regulatory bodies are discussing, or in some cases have created, a vaccine passport. This digital or paper document proves that its bearer has been fully vaccinated against COVID-19 — or perhaps has recently recovered from the disease or tested negative for it. Such documents in theory could help open borders and release travelers from quarantine requirements.

Can children get vaccinated for COVID-19?

It depends. The Pfizer vaccine currently can be given to kids 5 and older. But the Moderna and Johnson & Johnson vaccines still are authorized only for people 18 and older.

The Pfizer vaccine for 5- to 11-year-olds is one-third of the dose of the Pfizer vaccine that has been given to adults since December 2020 and adolescents 12 and up since May. Data presented to the FDA showed the new dose was 90.7% effective at preventing symptomatic COVID-19 in 5- to 11-year-olds. As with adults and adolescents, young children receiving the Pfizer vaccine will need a second dose three weeks later.

The Pfizer vaccine is being distributed through pediatricians' offices, pharmacies and other sites. Locations are listed at

Moderna recently asked the FDA for emergency use authorization for its vaccine in children ages 12 through 17.

Should I get the vaccine if I have a history of heart disease or stroke?

Yes. People with cardiovascular risk factors, heart disease, and heart attack and stroke survivors should get vaccinated(link opens in new window) as soon as possible because they are at much greater risk from the virus than they are from the vaccine, according to leaders of the American Heart Association. The AHA urges people with medical conditions to discuss vaccination with their health care team.

Does vaccination affect other medical procedures I might have? I've heard mammograms might be a problem.

If possible, get your mammogram before COVID-19 vaccination — or talk to your health care team about how long to wait afterward. (Some experts advise waiting four to six weeks.) As part of the normal immune reaction that occurs after COVID-19 vaccination, you might have lymph node swelling in the underarm near where you received your shot. The swelling might cause a false reading on your mammogram.

You should also consult with your health care team about the timing of your COVID-19 vaccination if you've recently been in the hospital or had surgery or another procedure requiring anesthesia — or if any of these are coming up.

Routine blood work and most other medical procedures and screenings can be performed as usual before or after getting your COVID-19 shot, the CDC says(link opens in new window). These include dental procedures, CT scans, electrocardiograms, cardiac stress tests, ultrasound and colonoscopy.

I have allergies. Should I get a COVID-19 vaccine?

If you've had a severe or immediate allergic reaction to any ingredient — including polyethylene glycol or polysorbate — in a particular type of COVID-19 vaccine, you should not get that vaccine (or not get it again). Ask your health care team whether you should get a different type of vaccine.

Also consult with your health care team if you've had any immediate allergic reaction to another vaccine or injectable treatment for another disease. If you have allergies that aren't related to vaccines, including food, pet or latex allergies, the CDC recommends you get vaccinated for COVID-19.

What if I have some other underlying condition?

The CDC has published extensive information on vaccination in different groups(link opens in new window), including in people with disabilities(link opens in new window) and those with underlying medical conditions(link opens in new window) such as HIV, autoimmune disorders and a history of Guillain-Barre syndrome.

If I already had COVID-19, do I still need to be vaccinated?

Yes. Because the virus that causes COVID-19 hasn't been around long, experts don't know how lasting people's immune protection is after infection. One recent study showed the risk of a repeat infection with the coronavirus was more than doubled in people who were not vaccinated.

If your COVID-19 infection was treated with monoclonal antibodies(link opens in new window) or convalescent plasma, a blood derivative from people who've been infected, the CDC urges you to wait 90 days before vaccination.

Wouldn't I be better protected against COVID-19 in the future if I just got infected naturally, instead of getting vaccinated?

There's no way to predict how COVID-19 would affect you. Severe COVID-19 illness, and even death, have occurred in people of all ages — including healthy people. And if you get infected, you could spread the illness to other vulnerable people.

By contrast, federal regulators have determined that the benefits of each vaccine outweigh any risks. The vaccines have been tested in thousands of people in clinical trials and continue to be monitored closely.

Along with COVID-19 precautions including face masks, physical distancing and handwashing, vaccination is considered an essential tool in stopping the pandemic and allowing people to safely return to normal activities.

Should I get a flu shot this year?

Yes. A yearly flu shot is recommended for everyone 6 months and older, with rare exceptions. For some young children, two doses are advised — check with your child’s health care team.

Influenza vaccination can help keep you and your family healthy this winter — so you might avoid a medical visit at a time when lots of people are sick and contagious. That would not only make you feel better, but it would help reduce the strain on health care systems already overtaxed by patients who have COVID-19 or other seasonal respiratory illnesses.

It’s particularly important to get a flu vaccine if you have a medical condition such as high blood pressure, diabetes or heart failure, or have had a heart attack or stroke.

Discuss vaccination with your health care team if you have allergies to eggs or flu shot ingredients, have had a severe allergic reaction to a previous influenza vaccine, have a history of Guillain-Barré syndrome or are feeling ill.

The flu vaccine will not protect you from COVID-19. So if you haven’t had a COVID-19 shot yet, you can get both vaccinations at the same time, the CDC says.

What does “herd immunity” mean?

Herd immunity refers to the point at which enough people in society have protective antibodies against an illness, either from having been vaccinated or previously infected, that it's hard for the disease to spread from person to person. Herd immunity helps protect people who are unable to be vaccinated, such as newborns or people who are allergic to the vaccine.

The percentage of people who need to be vaccinated to achieve herd immunity varies. For instance, with measles, one of the most contagious diseases, it's about 95%. Experts don't yet know how many people will have to be fully vaccinated against COVID-19 to achieve herd immunity but estimate that figure is upward of 70%.

What are virus “variants”? Can the current COVID-19 vaccines protect against them?

When a virus infects you, it hijacks your cells to make numerous copies of itself that can infect other cells. Errors, or mutations, occur during copying. A variant emerges when a copying error proves beneficial to the virus — for instance, by making it more infectious — and is carried forward. The more a virus circulates in a population, the more opportunity there is for mutations to occur and variants to emerge.

Scientists are still determining whether the coronavirus variants(link opens in new window) they've identified so far cause more severe illness and how easily they spread. Studies so far show that the vaccines now being administered can protect against at least some major variants, though scientists are continuing to investigate the question and will have to evaluate new variants as they emerge.

Where can I find out more about the COVID-19 vaccines?

The CDC website includes extensive, up-to-date information about COVID-19 vaccination(link opens in new window).

How can I take part in a clinical trial of a COVID-19 vaccine or treatment?

The website of the National Institute of Allergy and Infectious Diseases has information on clinical trials(link opens in new window).

What if I lose my COVID-19 vaccination card?

If you haven't taken a photo or made a photocopy of your card as a backup, then contact your vaccination provider. If you can't reach the provider directly, check with your state health department's immunization information system. (Vaccination providers are required to report COVID-19 shots to such record-keeping systems.) Also, if you enrolled in the CDC's side effects reporting system, v-safe, or its second-shot reminder program, VaxText, you can access personal vaccination information there.

HEALTH CARE DISCLAIMER: This site and its services do not constitute the practice of medical advice, diagnosis or treatment. Always talk to your health care provider for diagnosis and treatment, including your specific medical needs. If you have or suspect that you have a medical problem or condition, please contact a qualified health care professional immediately. If you are in the United States and experiencing a medical emergency, call 911 or call for emergency medical help immediately.