There are three main types of COVID-19 vaccines that are or soon will be undergoing large-scale (Phase 3) clinical trials in the United States. Below is a description of how each type of vaccine prompts our bodies to recognize and protect us from the virus that causes COVID-19. None of these vaccines can give you COVID-19.

• mRNA vaccines contain material from the virus that causes COVID-19 that gives our cells instructions for how to make a harmless protein that is unique to the virus. After our cells make copies of the protein, they destroy the genetic material from the vaccine. Our bodies recognize that the protein should not be there and build T-lymphocytes and B-lymphocytes that will remember how to fight the virus that causes COVID-19 if we are infected in the future.
• Protein subunit vaccines include harmless pieces (proteins) of the virus that cause COVID-19 instead of the entire germ. Once vaccinated, our immune system recognizes that the proteins don’t belong in the body and begins making T-lymphocytes and antibodies. If we are ever infected in the future, memory cells will recognize and fight the virus.
• Vector vaccines contain a weakened version of a live virus—a different virus than the one that causes COVID-19—that has genetic material from the virus that causes COVID-19 inserted in it (this is called a viral vector). Once the viral vector is inside our cells, the genetic material gives cells instructions to make a protein that is unique to the virus that causes COVID-19. Using these instructions, our cells make copies of the protein. This prompts our bodies to build T-lymphocytes and B-lymphocytes that will remember how to fight that virus if we are infected in the future.

New Approach to Vaccines

Within the next month, messenger RNA vaccines—also called mRNA vaccines—are likely to be some of the first COVID-19 vaccines authorized for use in the United States.

mRNA vaccines are a new type of vaccine to protect against infectious diseases. To trigger an immune response, many vaccines put a weakened or inactivated germ into our bodies. Not mRNA vaccines. Instead, they teach our cells how to make a protein—or even just a piece of a protein—that triggers an immune response inside our bodies. That immune response, which produces antibodies, is what protects us from getting infected if the real virus enters our bodies.

At the end of the process, our bodies have learned how to protect against future infection. The benefit of mRNA vaccines, like all vaccines, is those vaccinated gain this protection without ever having to risk the serious consequences of getting sick with COVID-19.

Sometimes after vaccination, the process of building immunity can cause symptoms, such as fever. These symptoms are normal and are a sign that the body is building immunity.

Most COVID-19 Vaccines Require More Than One Shot

All but one of the COVID-19 vaccines that are currently in Phase 3 clinical trials in the United States use two shots. The first shot starts building protection. A second shot a few weeks later is needed to get the most protection the vaccine has to offer. One vaccine in Phase 3 clinical trials only needs one shot.  It’s important to get the second dose for the COVID-19 vaccine to be effective.

After a vaccine is authorized or approved for use, many vaccine safety monitoring systems watch for adverse events (possible side effects). This continued monitoring can pick up on adverse events that may not have been seen in clinical trials. If an unexpected adverse event is seen, experts quickly study it further to assess whether it is a true safety concern. Experts then decide whether changes are needed in U.S. vaccine recommendations. This monitoring is critical to help ensure that the benefits continue to outweigh the risks for people who receive vaccines.

Expanded Safety Monitoring Systems

The following systems and information sources add an additional layer of safety monitoring, giving CDC and FDA the ability to evaluate COVID-19 vaccine safety in real time and make sure COVID-19 vaccines are as safe as possible

• CDC: V-SAFE pdf icon[644 KB, 21 pages] — A new smartphone-based, after-vaccination health checker for people who receive COVID-19 vaccines. V-SAFE will use text messaging and web surveys from CDC to check in with vaccine recipients for health problems following COVID-19 vaccination. The system also will provide telephone follow up to anyone who reports medically significant (important) adverse events.
• CDC: National Healthcare Safety Network (NHSN) — An acute care and long-term care facility monitoring system with reporting to the Vaccine Adverse Event Reporting System or VAERS
  FDA: Other large insurer/payer databases — A system of administrative and claims-based data for surveillance and research

After you are fully vaccinated for COVID-19, take these steps to protect yourself and others:

• Wear a mask with the best fit, protection, and comfort for you.
• If you are in an area with a high COVID-19 Community Level and are ages 2 or older, wear a mask indoors in public.
• If you are sick and need to be around others, or are caring for someone who has COVID-19, wear a mask.
• If you are at increased risk for severe illness, or live with or spend time with someone at higher risk, speak to your healthcare provider about wearing a mask at medium COVID-19 Community Levels.

No, vaccines cannot give someone COVID-19. None of the COVID-19 vaccines currently in development in the United States use the live virus that causes COVID-19. There are several different types of vaccines in development. However, the goal for each of them is to teach our immune systems how to recognize and fight the virus that causes COVID-19. Sometimes this process can cause symptoms, such as fever. These symptoms are normal and are a sign that the body is building immunity.

It typically takes a few weeks for the body to build immunity after vaccination. That means it’s possible a person could be infected with the virus that causes COVID-19 just before or just after vaccination and get sick. This is because the vaccine has not had enough time to provide protection.

Yes, you should be vaccinated regardless of whether you already had COVID-19 because:

• Research has not yet shown how long you are protected from getting COVID-19 again after you recover from COVID-19.
• Vaccination helps protect you even if you’ve already had COVID-19.

Evidence is emerging that people get better protection by being fully vaccinated compared with having had COVID-19. One study showed that unvaccinated people who already had COVID-19 are more than 2 times as likely than fully vaccinated people to get COVID-19 again.

If you were treated for COVID-19 with monoclonal antibodies or convalescent plasma, you should wait 90 days before getting a COVID-19 vaccine. Talk to your doctor if you are unsure what treatments you received or if you have more questions about getting a COVID-19 vaccine.

If you or your child has a history of multisystem inflammatory syndrome in adults or children (MIS-A or MIS-C), consider delaying vaccination until you or your child have recovered from being sick and for 90 days after the date of diagnosis of MIS-A or MIS-C. Learn more about the clinical considerations for people with a history of multisystem MIS-C or MIS-A.

Experts are still learning more about how long vaccines protect against COVID-19. CDC will keep the public informed as new evidence becomes available.

COVID-19 Vaccine Primary Series

The number of vaccine doses you need depends on which vaccine you receive.

• Two doses of Pfizer-BioNTech vaccine should be given 3 weeks (21 days) apart.
• Two doses of Moderna vaccine should be given 4 weeks (28 days) apart.
• Only one dose of Johnson & Johnson’s Janssen (J&J/Janssen) vaccine should be given.

If you receive a vaccine that requires two doses, you should get your second shot as close to the recommended interval as possible. You should not get the second dose earlier than the recommended interval.

COVID-19 vaccines are not interchangeable for your COVID-19 vaccine primary series.

If you received a Pfizer-BioNTech or Moderna COVID-19 vaccine for your first shot, you should get the same product for your second shot.

Additional Primary Dose If You Are Immunocompromised

If you received a Pfizer-BioNTech (ages 12 and older) or Moderna (ages 18 and older) mRNA COVID-19 vaccine primary series and have a moderately to severely compromised immune system, you should receive an additional primary dose of the same mRNA COVID-19 vaccine at least 28 days after the second dose.

Additional primary doses are not interchangeable. The vaccine used for the additional primary dose should be same as the vaccine used for the primary vaccine series. If the mRNA vaccine product given for the first two doses is not available or is unknown, either mRNA COVID-19 vaccine product may be administered.

Booster Shot

The CDC recommends everyone ages 5 years and older receive a COVID-19 vaccine booster after completing their primary COVID-19 vaccination series. Adults 50 years or older are eligible for a second booster shot.

If you get a booster shot you have the option to either get the same COVID-19 vaccine product as your primary series, or you can get a different COVID-19 vaccine. You may have a preference for the vaccine type that you originally received, and you may prefer to get a different booster. CDC’s recommendations now allow for this type of mix and match dosing for booster shots (Pfizer-BioNTech, Moderna, or J&J/Janssen). You may consider the benefits and risks of each product and discuss with your healthcare provider which COVID-19 vaccine product is the most appropriate booster for you.

There are no known fertility impacts from the COVID vaccine. It’s worth noting that in both the Pfizer and Moderna trials, which checked for pregnancy often and required all women to be on birth control, about as many women became pregnant in the vaccine sides of the trials than the placebo sides. In the Pfizer study, 12 women became pregnant in the vaccine group and had no known adverse effects in their pregnancies. Eleven women became pregnant in the placebo group and two miscarried of that group (which is about a normal miscarriage rate).

In the Moderna study, six women in the vaccine group and seven in the placebo group became pregnant. In the vaccine group, there were no reported problems in the pregnancies. Of the seven in the placebo group, one had an abortion and one miscarried.

The CDC notes that ‘No safety concerns were demonstrated in rats that received Moderna COVID-19 vaccine prior to or during gestation in terms of female reproduction, fetal/embryonal development, or postnatal development,’ and ‘There is no recommendation for routine pregnancy testing before receipt of a COVID-19 vaccine. Those who are trying to become pregnant do not need to avoid pregnancy after mRNA COVID-19 vaccination.’

Yes, COVID-19 vaccination is recommended for all people 12 years and older, including people who are pregnant, breastfeeding, trying to get pregnant now, or might become pregnant in the future. You might want to have a conversation with your healthcare provider about COVID-19 vaccination. While such a conversation might be helpful, it is not required before vaccination. Learn more about vaccination considerations for people who are pregnant or breastfeeding.

If you are pregnant and have received a COVID-19 vaccine, we encourage you to enroll in v-safe, CDC’s smartphone-based tool that provides personalized health check-ins after vaccination. A v-safe pregnancy registry has been established to gather information on the health of pregnant people who have received a COVID-19 vaccine.

Vaccines currently in clinical trials in the United States will not cause you to test positive on viral tests, which are used to see if you have a current infection.

If your body develops an immune response, which is the goal of vaccination, there is a possibility you may test positive on some antibody tests. Antibody tests indicate you had a previous infection and that you may have some level of protection against the virus. Experts are currently looking at how COVID-19 vaccination may affect antibody testing results.