COVID-19 vaccines: Get the facts

Looking to get the facts about the new COVID-19 vaccines? Here's what you need to know about the different vaccines and the benefits of getting vaccinated.

Vaccines to prevent coronavirus disease 2019 (COVID-19) are perhaps the best hope for ending the pandemic. But as the U.S. Food and Drug Administration (FDA) continues authorizing emergency use of COVID-19 vaccines, you likely have questions. Find out about the benefits of the COVID-19 vaccines, how they work, the possible side effects and the importance of continuing to take infection prevention steps.

What are the benefits of getting a COVID-19 vaccine?

A COVID-19 vaccine might:

  • Prevent you from getting COVID-19 or from becoming seriously ill or dying due to COVID-19
  • Prevent you from spreading the COVID-19 virus to others
  • Add to the number of people in the community who are protected from getting COVID-19 — making it harder for the disease to spread and contributing to herd immunity
  • Prevent the COVID-19 virus from spreading and replicating, which allows it to mutate and possibly become more resistant to vaccines

What COVID-19 vaccines have been authorized and how do they work?

Currently, several COVID-19 vaccines are in clinical trials. The FDA will review the results of these trials before approving COVID-19 vaccines for use. But because there is an urgent need for COVID-19 vaccines and the FDA's vaccine approval process can take months to years, the FDA will first be giving emergency use authorization to COVID-19 vaccines based on less data than is normally required. The data must show that the vaccines are safe and effective before the FDA can give emergency use authorization. Vaccines with FDA emergency use authorization include:

  • Pfizer-BioNTech COVID-19 vaccine. The Pfizer-BioNTech COVID-19 vaccine is 95% effective in preventing the COVID-19 virus with symptoms in people age 16 and older. The vaccine is 100% effective in preventing the COVID-19 virus in children ages 12 through 15. This vaccine is for people age 12 and older. It requires two injections given 21 days apart. The second dose can be given up to six weeks after the first dose, if needed.
  • Moderna COVID-19 vaccine. The Moderna COVID-19 vaccine is 94% effective in preventing the COVID-19 virus with symptoms. This vaccine is for people age 18 and older. It requires two injections given 28 days apart. The second dose can be given up to six weeks after the first dose, if needed.
  • Janssen/Johnson & Johnson COVID-19 vaccine. In clinical trials, this vaccine was 66% effective in preventing the COVID-19 virus with symptoms — as of 14 days after vaccination. The vaccine also was 85% effective at preventing severe disease with the COVID-19 virus — at least 28 days after vaccination. This vaccine is for people age 18 and older. It requires one injection. The FDA and the Centers for Disease Control and Prevention (CDC) have recommended that use of this vaccine continue in the U.S. because the benefits outweigh the risks. If you are given this vaccine, you should be educated about the possible risks and symptoms of a blood clotting problem.

Both the Pfizer-BioNTech and the Moderna COVID-19 vaccines use messenger RNA (mRNA). Coronaviruses have a spikelike structure on their surface called an S protein. COVID-19 mRNA vaccines give cells instructions for how to make a harmless piece of an S protein. After vaccination, your cells begin making the protein pieces and displaying them on cell surfaces. Your immune system will recognize that the protein doesn't belong there and begin building an immune response and making antibodies.

The Janssen/Johnson & Johnson COVID-19 vaccine is a vector vaccine. In this type of vaccine, genetic material from the COVID-19 virus is inserted into a different kind of weakened live virus, such as an adenovirus. When the weakened virus (viral vector) gets into your cells, it delivers genetic material from the COVID-19 virus that gives your cells instructions to make copies of the S protein. Once your cells display the S proteins on their surfaces, your immune system responds by creating antibodies and defensive white blood cells. If you become infected with the COVID-19 virus, the antibodies will fight the virus.

Viral vector vaccines can't cause you to become infected with the COVID-19 virus or the viral vector virus. Also, the genetic material that's delivered doesn't become part of your DNA.

Are the new COVID-19 vaccines safe?

Andrew Badley, M.D., COVID-19 Research Task Force Chair, Mayo Clinic: The safety of these vaccines has been studied extensively. They've been tested now in about 75,000 patients in total, and the incidence of adverse effects is very, very low.

These vaccines were fast-tracked, but the parts that were fast-tracked were the paperwork; so the administrative approvals, the time to get the funding — those were all fast-tracked. Because these vaccines have such great interest, the time it took to enroll patients was very, very fast. The follow up was as thorough as it is for any vaccine, and we now have months of data on patients who received the vaccine or placebo, and we've compared the incidence of side effects between patients who received the vaccine and placebo, and that incidence of side effects, other than injection site reaction, is no different.

The side effects to the vaccines are very mild. Some of them are quite common. Those include injection site reactions, fevers, chills, and aches and pains. In a very, very small subset of patients — those patients who've had prior allergic reactions — some patients can experience allergic reaction to the vaccine. Right now we believe that number is exceedingly low.

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Are COVID-19 vaccines free?

In the U.S., the federal government is providing COVID-19 vaccines free of charge to all residents, regardless of immigration status or health insurance coverage. The COVID-19 vaccines are free whether offered by a clinic, retail pharmacy or other location.

If children don't frequently experience severe illness with COVID-19, why do they need a COVID-19 vaccine?

A COVID-19 vaccine can prevent your child from getting and spreading the COVID-19 virus. If your child gets COVID-19, a COVID-19 vaccine could prevent him or her from becoming severely ill.

Getting a COVID-19 vaccine will also allow your child to start doing things that he or she might not have been able to do because of the pandemic, including not wearing a mask or social distancing in any setting — except where required by a rule or law.

How did the FDA determine the safety and effectiveness of the Pfizer-BioNTech COVID-19 vaccine for use in kids ages 12 through 15?

The FDA reviewed a study of more than 2,200 U.S. children ages 12 through 15. Of this group, about half were given the Pfizer-BioNTech COVID-19 vaccine. The other children were given a harmless, fake (placebo) shot.

A week after the second dose was given, research showed no cases of COVID-19 in the 1,005 children given the Pfizer-BioNTech vaccine. Among 978 children given the placebo, there were 16 cases of COVID-19. None of the children had previously been diagnosed with COVID-19. The results suggest that the vaccine is 100% effective at preventing the COVID-19 virus in this age group.

A portion of the children given the vaccine were also monitored for at least two months after being given the second dose.

What is the concern about the Janssen/Johnson & Johnson COVID-19 vaccine?

Use of the Janssen/Johnson & Johnson COVID-19 vaccine might increase the risk of a rare and serious blood clotting disorder. Nearly all of those affected have been women ages 18 to 49, with the disorder happening at a rate of 7 for every 1 million vaccinated women in this age group. For women age 50 and older and men of all ages, the disorder is even more rare.

The FDA and the CDC have recommended that use of the vaccine in the U.S. can continue because the benefits outweigh the risks. If you are given this vaccine, you should be educated about the possible risks and symptoms of a blood clotting problem.

What are the symptoms of a blood clotting reaction to the Janssen/Johnson & Johnson COVID-19 vaccine?

Serious side effects of the Janssen/Johnson & Johnson COVID-19 vaccine can occur within three weeks of vaccination and require emergency care. Possible symptoms include:

  • Shortness of breath
  • Persistent stomach pain
  • Severe or persistent headaches or blurred vision
  • Chest pain
  • Leg swelling
  • Easy bruising or tiny red spots on the skin beyond the injection site

Mild to moderate headaches and muscle aches are common in the first three days after vaccination and don't require emergency care.

Do the COVID-19 vaccines protect against the COVID-19 variants?

The COVID-19 vaccines were developed based on the S protein before it contained the mutations identified in the variants. While research suggests that COVID-19 vaccines have lower efficacy against the variants, the vaccines still appear to provide protection against severe COVID-19. Further research is needed.

In addition, vaccine manufacturers are also creating booster shots to improve protection against variants.

Can a COVID-19 vaccine give you COVID-19?

No. The COVID-19 vaccines currently being developed in the U.S. don't use the live virus that causes COVID-19.

Keep in mind that it will take a few weeks for your body to build immunity after getting a COVID-19 vaccination. As a result, it's possible that you could become infected with the virus that causes COVID-19 just before or after being vaccinated.

What are the possible side effects of a COVID-19 vaccine?

A COVID-19 vaccine can cause mild side effects after the first or second dose, including:

  • Pain, redness or swelling where the shot was given
  • Fever
  • Fatigue
  • Headache
  • Muscle pain
  • Chills
  • Joint pain
  • Nausea and vomiting
  • Feeling unwell
  • Swollen lymph nodes

You'll likely be monitored for 15 minutes after getting a COVID-19 vaccine to see if you have an immediate reaction. Most side effects happen within the first three days after vaccination and typically last only one to two days.

Serious side effects of the Janssen/Johnson & Johnson COVID-19 vaccine can occur within three weeks of vaccination and require emergency care. Possible symptoms include:

  • Shortness of breath
  • Persistent stomach pain
  • Severe or persistent headaches or blurred vision
  • Chest pain
  • Leg swelling
  • Easy bruising or tiny red spots on the skin beyond the injection site

A COVID-19 vaccine may cause side effects similar to signs and symptoms of COVID-19. If you've been exposed to COVID-19 and you develop symptoms more than three days after getting vaccinated or the symptoms last more than two days, self-isolate and get tested.


What are the signs of an allergic reaction to a COVID-19 vaccine?

You might be having an allergic reaction to a COVID-19 vaccine if you experience these signs within four hours of your first vaccine dose:

  • Continuous shortness of breath or wheezing
  • Swelling of the lips, eyes or tongue
  • Redness, swelling or itchiness in areas of the body other than the limb in which the vaccine was given

If you have any signs of an allergic reaction, get help right away. Tell your doctor about your reaction, even if it went away on its own or you didn’t get emergency care. This reaction might mean you are allergic to the vaccine. You might not be able to get a second dose of the same vaccine. However, you might be able to get a different vaccine for your second dose.

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Is it OK to take an over-the-counter pain medication before or after getting a COVID-19 vaccine?

It isn’t recommended that you take a pain medication before getting a COVID-19 vaccine to prevent possible discomfort. It’s not clear how these medications might impact the effectiveness of the vaccines. However, it’s OK to take this kind of medication after getting a COVID-19 vaccine, as long as you have no other medical reason that would prevent you from taking it.

What are the long-term side effects of the COVID-19 vaccines?

Because COVID-19 vaccines clinical trials only started in the summer of 2020, it’s not yet clear if these vaccines will have long-term side effects. However, vaccines rarely cause long-term side effects.

If you're concerned, in the U.S., safety data on COVID-19 vaccines will be reported to a national program called the Vaccine Adverse Event Reporting System. This data is available to the public. The CDC has also created v-safe, a smartphone-based tool that allows users to report COVID-19 vaccine side effects.

Can I get a COVID-19 vaccine if I have a history of allergic reactions?

If you have a history of severe allergic reactions not related to vaccines or injectable medications, you may still get a COVID-19 vaccine. You should be monitored for 30 minutes after getting the vaccine.

If you've had an immediate allergic reaction to other vaccines or injectable medications, ask your doctor if you should get a COVID-19 vaccine. If you’ve ever had an immediate or severe allergic reaction to any ingredient in a COVID-19 vaccine, the CDC recommends not getting that specific vaccine.

If you have an immediate or severe allergic reaction after getting the first dose of a COVID-19 vaccine, don't get the second dose. However, you might be able to get a different vaccine for your second dose.

Can I get a COVID-19 vaccine if I have an existing health condition?

Yes, if you have an existing health condition you can get a COVID-19 vaccine — as long as you haven't had an allergic reaction to a COVID-19 vaccine or any of its ingredients. But there is limited information about the safety of the COVID-19 vaccines in people who have weakened immune systems or autoimmune conditions.

Can pregnant or breastfeeding women get the COVID-19 vaccine?

If you are pregnant or breastfeeding, you may choose to get a COVID-19 vaccine. While further research is needed, early findings suggests that getting an mRNA COVID-19 vaccine during pregnancy poses no serious risks. The findings are based on data from the CDC’s coronavirus vaccine safety monitoring system.

If you have concerns, talk to your health care provider about the risks and benefits.

Keep in mind that the mRNA COVID-19 vaccines don’t alter your DNA or cause genetic changes.

Can a COVID-19 vaccine affect fertility or menstruation?

There is currently no evidence that any COVID-19 vaccines cause fertility problems.

A small number of women have reported experiencing temporary menstrual changes after getting a COVID-19 vaccine. A small study has also shown that some women experienced temporary menstrual changes after getting COVID-19. It's not clear if getting COVID-19 or a COVID-19 vaccine causes these changes. Further research is needed.

Keep in mind that many things can affect menstrual cycles, including infections, stress, sleep problems and changes in diet or exercise.

Is there anyone who should not get a COVID-19 vaccine?

There is no COVID-19 vaccine yet for children under age 12. Clinical trials involving younger children are in progress.

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

Getting COVID-19 might offer some natural protection or immunity from reinfection with the virus that causes COVID-19. But it's not clear how long this protection lasts. Because reinfection is possible and COVID-19 can cause severe medical complications, it's recommended that people who have already had COVID-19 get a COVID-19 vaccine. If you were treated for COVID-19 with monoclonal antibodies or convalescent plasma, wait 90 days before getting a COVID-19 vaccine.

Can I stop taking safety precautions after getting a COVID-19 vaccine?

You are considered fully vaccinated 2 weeks after you get a second dose of an mRNA COVID-19 vaccine or 2 weeks after you get a single dose of the Janssen/Johnson & Johnson COVID-19 vaccine. After you are fully vaccinated, you can return to doing activities that you might not have been able to do because of the pandemic. You can also stop wearing a mask or social distancing in any setting, except where required by a rule or law.

However, you will still be required to wear a mask on planes, buses, trains and other public transportation traveling to, within, or out of the U.S., as well as in places such as airports and train stations.

If you are traveling in the U.S., you don’t need to get tested before or after your trip or quarantine after you return. If you are traveling outside of the U.S., you don’t need to get tested before you leave the U.S. unless your destination requires it. You still need to show a negative test result or proof that you’ve recovered from COVID-19 in the past 3 months before boarding an international flight to the U.S. It’s also recommended that you get tested 3 to 5 days after international travel. However, quarantining isn't needed.

If you've been around someone who has COVID-19, you don’t need to stay away from others or get tested unless you have symptoms of COVID-19. But if you are a resident or employee of a correctional or detention facility or a homeless shelter and are around someone who has COVID-19, you should still get tested even if you don’t have symptoms of COVID-19.

Can I still get COVID-19 after I’m vaccinated?

COVID-19 vaccination will protect most people from getting sick with COVID-19.

A very small percentage of fully vaccinated people will still get COVID-19 if they are exposed to the COVID-19 virus. These are called vaccine breakthrough cases. Some people might not experience any symptoms and some people could become sick due to COVID-19.

However, vaccination might make illness less severe. If you are fully vaccinated, the overall risk of hospitalization and death due to COVID-19 is much lower than among unvaccinated people with similar risk factors.

June 12, 2021 See more In-depth

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