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.?

For most of the pandemic three vaccines(link opens in new window), commonly referred to by the names of the companies that make them, have been available in the United States as primary and booster vaccinations:

The Pfizer and Moderna vaccines are most highly recommended, while the Johnson & Johnson vaccine is recommended only in some cases.

A fourth vaccine, made by Novavax(link opens in new window), was authorized for use as a two-dose series in people ages 12 and older in summer 2022.

Federal authorities also have authorized updated formulations of the Pfizer and Moderna vaccinations for use as booster shots. These “bivalent” vaccines are designed to target both the original strain of the virus that causes COVID-19 plus recent dominant subvariants of the virus — lineages of the omicron variant. They are called bivalent COVID-19 vaccines because they include a component of the original virus strain and a component of the virus’s omicron variant.

Each of these new booster formulations is authorized as a single dose two months or more after receiving an initial (“primary”) vaccination series or the most recent non-bivalent booster. The updated Moderna booster is for people 6 months and older, while the Pfizer shot is available for anyone age 5 years and up.

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, and Novavax is a protein-based (or protein-subunit) vaccine.(link opens in new window) All these 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 help protect you from getting COVID-19, although fully vaccinated people can get infected. (That’s called a “breakthrough infection(link opens in new window).”) But more importantly, vaccination has been shown to greatly reduce the risk of getting seriously ill and being hospitalized if you do get COVID-19.

I’m not vaccinated yet. Where do I start?

Vaccine availability, dose and recommended number of shots vary depending on the type of vaccine you get and your age and immune status.

The first round of COVID-19 shots is called your “primary series” of vaccinations. If you’re an adult, in most cases you would start with two doses of the Pfizer vaccine (three to eight weeks apart), two doses of the Moderna vaccine (four to eight weeks apart) or two doses of the Novavax vaccine (three to eight weeks apart). For the primary series, experts recommend you get the same brand of shot each time. Your health care team can guide you on whether it’s best to get the second shot as soon as you’re eligible (particular for people 65 and older) or to wait the full eight weeks for the second Pfizer or Moderna vaccination.

Some people require three doses for their primary series — including children ages 6 months through 4 years receiving the Pfizer vaccine and people of any age with markedly weakened immune systems receiving the Pfizer or Moderna shots.

The Johnson & Johnson Janssen vaccine is not recommended except in unusual cases — for instance, if you’re allergic to any ingredients in the Pfizer or Moderna vaccines or if those vaccines are not available to you. The primary series is one dose.

The CDC provides details(link opens in new window) about the vaccination schedule for each COVID-19 vaccine used in the U.S. Visit opens in new window) to find the vaccination you (or your family members) need near you.

How long will the vaccine protect me?

Scientists are studying this question, although research has indicated that the level of protection wanes over time — leading public health authorities to recommend that most people who have completed their primary vaccination series for COVID-19 later receive a bivalent booster shot.

When should I get a first or second booster shot? Which vaccine should I get?

The CDC offers detailed advice(link opens in new window) on vaccination and booster shots in different groups, including a helpful tool to guide you on getting booster shots for you or your child. (Click on “Find Out When to Get a Booster.”) Note that recommendations vary if you were originally vaccinated abroad.

How do I know if I’m ‘immunocompromised’?

Conditions such as advanced HIV infection and some genetic disorders can compromise, or weaken, 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.

Your health care team can guide you on whether you have a weakened immune system and should receive additional vaccinations. 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 does ‘fully vaccinated’ mean? When am I ‘up to date’?

The CDC considers people fully vaccinated two weeks after receiving the second dose in a two-dose primary series (Pfizer or Moderna) or the initial single dose of the Johnson & Johnson Janssen vaccine. Up to date means they’ve received their primary vaccination series and all recommended boosters when they are eligible for them. Note that three shots are recommended in the primary series of vaccinations for some groups, including children up to age 4 receiving the Pfizer vaccine and people who are immunocompromised receiving the Pfizer or Moderna vaccine.

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

All the vaccines available in the U.S. have initially 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 primary series of the Pfizer vaccine became the first fully approved COVID-19 immunization for people ages 16 and older; in July 2022 it was additionally approved for children ages 12 to 15. In January 2022, the Moderna vaccine received full approval for primary vaccination in people 18 and older. These vaccines also have emergency use authorization for other groups. The Johnson & Johnson Janssen vaccine is available (on a more limited basis(link opens in new window)) under emergency use authorization.

As of December 2022, the FDA had issued emergency use authorizations for the fourth vaccine, Novavax(link opens in new window), to prevent COVID-19 in people 12 and older.

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

Two serious medical conditions have been associated with the J&J vaccine – a neurological condition called Guillain-Barré syndrome and a clotting disorder called thrombosis-thrombocytopenia syndrome.

In Guillain-Barré, the immune system attacks nerve cells, causing muscle weakness or sometimes paralysis. Other 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.

Although a rare occurrence, Guillain-Barré has been diagnosed in patients more frequently in the wake of vaccination with the J&J vaccine, compared with the Pfizer or Moderna shots. Most people fully recover from Guillain-Barré, the CDC reports.

The other condition, thrombosis-thrombocytopenia syndrome (TTS), is linked to about four cases per every million doses given of the J&J vaccine. TTS is a potentially deadly disorder that 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.

People who get the J&J 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.

Are these conditions a problem with the other vaccines?

Neither of these conditions, TTS or Guillain-Barré, has been associated with the Pfizer and Moderna vaccines — and over 1 billion doses of those have been given so far. However, there have been rare reports of inflammatory conditions called myocarditis and pericarditis(link opens in new window) with these vaccines and with the new Novavax shot (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 about 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 or difficulty breathing. If you have any of these allergic reactions, the CDC recommends(link opens in new window) you not get a second shot of that type of vaccine.

In addition, public health experts are monitoring(link opens in new window) 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.)

Cases of myocarditis and/or pericarditis have been more common in adolescent boys and young men. 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.

Side effects can be reported to the CDC through v-safe(link opens in new window), a smartphone-based tool.

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

Yes — and get up to date on booster shots, too. 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 not your first shot, bring the CDC COVID-19 Vaccination Record Card(s) that you received at previous visits. 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 (see “side effects” question above), does not occur. Health care workers onsite are ready to treat anyone who has this reaction.

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.

Where can I get the vaccine?

Check with your local pharmacies — many are part of a federal program(link opens in new window) to increase vaccine access. You can also use the VaccineFinder tool(link opens in new window) to search for vaccine providers in your area, or check the website of your state or territorial health department(link opens in new window). 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 a two-shot primary 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(link opens in new window).

You can report side effects directly to the CDC through a smartphone-based tool called v-safe.(link opens in new window) (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).

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(link opens in new window), including physical distancing and masking in communities where COVID-19 rates(link opens in new window) are high.

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 need a test for travel, work, school, or in advance of an event or social gathering
  • 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.

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

Research so far has suggested the vaccines can help limit the spread of COVID-19 (in addition to preventing severe illness or death in you). However, new variants continue to challenge the vaccines’ ability to prevent infection.

Wait a minute – isn’t the pandemic over? Can’t I just go back to my old lifestyle?

The term pandemic refers to the spread of a disease across several countries, affecting a large number of people. By that definition, the COVID-19 pandemic continues globally(link opens in new window).

In the U.S., some counties have "medium" or "high" levels of COVID-19 transmission in the community (check your local rates with this COVID Data Tracker(link opens in new window)). In areas with high transmission, the CDC advises everyone to wear a high-quality mask(link opens in new window) or respirator in public settings and people at high risk of getting sick to consider avoiding nonessential indoor activities in public. (More guidance for different levels of community transmission is here(link opens in new window)).

Masks are still required for fully vaccinated people in some circumstances, such as where state or local law mandates them, and in health care settings, businesses and workplaces that require masking.

Traveling is more complicated. Wearing a well-fitting mask over your nose and mouth is recommended(link opens in new window) in indoor areas of public transportation (including airplanes, trains and buses) and indoors in U.S. transportation hubs (including airports). The CDC provides detailed guidance for domestic(link opens in new window) and international travel.

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.

Can children get vaccinated for COVID-19?

Yes. The CDC recommends everyone ages 6 months and older get fully vaccinated against COVID-19. Boosters are also advised for some children – check out the CDC’s booster tool or consult with your pediatric care team.

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 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 ill or have a major medical procedure scheduled soon.

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

If you've had a severe or immediate allergic reaction to any of the COVID-19 vaccines’ ingredients (listed by vaccine type here(link opens in new window)) — including polyethylene glycol or polysorbate — 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.

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

Yes – vaccination will provide extra protection against another case of COVID-19. Because reinfection is less likely during a short window after having COVID-19, you may consider delaying your vaccine by three months from when symptoms started or when you tested positive. But you should take into account your personal risk and rates of COVID-19 in your community, the CDC says.

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. If you do get infected, vaccination helps protect you against severe illness, hospitalization and death.

Should I get a flu shot this fall?

Yes. A yearly flu shot is recommended for everyone 6 months and older, with rare exceptions. For some young children(link opens in new window), 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. Flu vaccination is particularly important for children age 2 and under and adults 65 and older. It’s also vital for who have any of a variety of medical conditions(link opens in new window) including kidney disease, diabetes or heart failure; have a body mass index of 40 or higher; 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(link opens in new window) 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 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.

Variants to the virus that causes COVID-19 so far have included Alpha, Delta and the one now of most consequence, Omicron. Subvariants, or lineages, of Omicron are currently dominant(link opens in new window) in the U.S.

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.

Experts in infectious diseases now say(link opens in new window) achieving herd immunity against the virus that causes COVID-19 is unlikely. That’s because, among other factors, the virus is continually mutating, creating new variants; it can be spread by people without symptoms (making it harder to track and control); rates of vaccination and use of other precautionary measures in the population are inadequate; and protection against new COVID-19 infection provided by previous infection or vaccination isn’t lasting.

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(link opens in new window). (Vaccination providers are required to report COVID-19 shots to such record-keeping systems.) The CDC provides more about your vaccination card.

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.