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. This vaccine is for people age 16 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 a pause in distributing this vaccine due to rare blood clotting reactions in a small number of people who have gotten the vaccine.

Both the Pfizer-BioNTech and the Moderna COVID-19 vaccines use messenger RNA (mRNA). Coronaviruses have a spike-like 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.

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

A rare and severe blood clotting reaction has occurred in a small number of the nearly 7 million people who have gotten the Janssen/Johnson & Johnson COVID-19 vaccine in the U.S. These reactions happened six to 13 days after vaccination. As a result, the FDA and CDC have recommended a pause in distributing this vaccine. Evidence of these blood clots haven’t been reported in the Pfizer-BioNTech or Moderna COVID-19 vaccines.

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

If you received the Janssen/Johnson & Johnson vaccine within the last three weeks and are experiencing any unexplained new severe symptoms, seek emergency care. Possible symptoms include:

  • Shortness of breath
  • Stomach pain
  • Severe headache
  • New neurological symptoms
  • Leg pain or swelling
  • Severe backache
  • Unexpected bruising
  • Tiny red spots on the skin

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
  • Stomach pain
  • Severe headache
  • New neurological symptoms
  • Leg pain or swelling
  • Severe backache
  • Unexpected bruising
  • Tiny red spots on the skin

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.

How are the COVID-19 vaccines being distributed?

In the U.S., the CDC has recommended that the COVID-19 vaccines first be offered to:

  • Health care personnel
  • Adult residents of long-term care facilities
  • Frontline essential workers, such as first responders and teachers
  • People age 75 and older
  • People ages 65 to 74
  • People ages 16 to 64 with existing health conditions
  • Other essential workers, such as people who work in food service and construction

Guidelines for who will be vaccinated first also vary by state in the U.S. Consult your local health department for the latest information on how and when you can receive a vaccine.

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?

There is no research on the safety of COVID-19 vaccines in pregnant or breastfeeding women. However, if you are pregnant or breastfeeding and part of a group recommended to get a COVID-19 vaccine, you may choose to get the vaccine. Talk to your health care provider about the risks and benefits.

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

There is no COVID-19 vaccine yet for children under age 16. Several companies have begun enrolling children as young as age 12 in COVID-19 vaccine clinical trials. Studies including younger children have also begun.

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’ve had COVID-19, you might delay vaccination until 90 days after your diagnosis. Reinfection with the virus that causes COVID-19 is uncommon in the 90 days after you are first infected.

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

After getting a COVID-19 vaccine, the CDC recommends that it’s OK for fully vaccinated people to:

  • Visit other fully vaccinated people indoors — without wearing masks or avoiding close contact
  • Visit unvaccinated people from one household who are at low risk for severe illness from COVID-19 — indoors and without wearing masks or avoiding close contact

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.

However, vaccinated people should continue to take safety precautions, such as wearing a mask and avoiding close contact (within about 6 feet, or 2 meters) with others, when they are:

  • In public
  • Visiting people who are unvaccinated and at increased risk for severe illness from COVID-19
  • Visiting people who have an unvaccinated household member at increased risk for severe illness from COVID-19
  • Visiting unvaccinated people from many households

Keep in mind that if you’re fully vaccinated from COVID-19, your risk of getting COVID-19 might be low. But if you become infected, you might spread the COVID-19 virus to others even if you don’t have any signs or symptoms of COVID-19.

April 16, 2021 See more In-depth

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