If you got the first dose of the Moderna or Pfizer COVID-19 vaccine — or expect to get it soon — you may have questions about the waiting period between the first and second dose. Both vaccines require two doses, but they have different waiting periods.

People get the second Pfizer dose three weeks after the first dose and the second Moderna dose four weeks after the first. Here’s what happens in your body during and after those waiting periods.

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How Do Multi-Dose Vaccines Work?

Multi-dose vaccines are ones that require more than one dose before the body is fully protected against a disease.

Examples of other multi-dose vaccines include those against polio, measles, hepatitis A, and hepatitis B. Each has an initial dose and then requires two or three additional doses for a person to have full protection.

The first dose of these vaccines introduces your body to either a dead or weakened form of the virus, or a small piece of the virus. That introduction causes your body to begin making antibodies that can fight that particular virus.

With the second dose, your body ramps up antibody production so it has enough in case the real virus ever enters your body.

COVID-19 vaccines are mRNA vaccines and do not contain SARS-CoV-2 (the virus that causes COVID-19)or even a piece of it. But they do contain instructions for making the spike protein, a protein found on the SARS-CoV-2 virus. The first dose leads your cells to make the spike protein, which then causes your body to make antibodies against it.

It takes time for your cells to make the protein and for your body to create antibodies. This is why there is a waiting period between the first and second doses. After the body has enough time to start making antibodies, the second dose amps up the response to create even more antibodies.

How Do Scientists Determine Vaccine Waiting Periods?

Different vaccines require different waiting periods based on the type of vaccine, how it’s made, and how and when the immune system responds.

Researchers discover the best spacing between doses during the early phases of vaccine clinical trials. During phases 1 and 2 of the trials, researchers test different spacing options to see which waiting period offers the best response.

The Pfizer and Moderna vaccines have different waiting periods because the clinical trials showed the best spacing for each one.

Scientists continue to study the effectiveness of the vaccine with different waiting periods after the public begins receiving them. They have already learned that people can receive the second Pfizer or Moderna dose up to 42 days after the first.

If you can’t get your second dose 21 days (for Pfizer) or 28 days (for Moderna) after your first dose, it is ideal to get the second dose within 42 days. But you can still get the second dose if you exceed the 42-day period. And if you get your second dose too early or too late, you don’t have to restart the vaccination process.

Differences and Similarities With Childhood Vaccines

Many people familiar with childhood vaccinations know that children often receive several vaccines at once.

Most childhood vaccines are multi-dose vaccines, which require children to receive additional doses of those vaccines at different times.

Because children require protection again many diseases, they typically receive multiple vaccines (for different diseases) simultaneously.

For example, children receive three doses of the vaccine against pertussis, tetanus, and diphtheria (DTaP) to be fully vaccinated against those diseases. The first dose is given at 2 months old, the second at 4 months, and the third at 6 months. They receive boosters at 15 to 18 months and 4 to 6 years, but boosters are not part of the original three-dose vaccination plan

But children receive the DTaP at the same time as other vaccines, such as ones against polio and rotavirus.

With the COVID-19 vaccines, adults are only receiving a vaccine against one disease — COVID-19. Scientists are studying whether adults can receive a COVID-19 vaccine at the same time as another vaccine, such as a flu shot.

Childhood vaccines are given together because researchers tested each one with others to make sure that it is safe and effective to give multiple vaccines at once. Some childhood vaccines have a higher risk of side effects if given at the same time. For example, there’s a higher chance of a fever or rash when children receive the measles-mumps-rubella (MMR) and chickenpox vaccines together.

It is too soon to know if getting the COVID-19 vaccine with any other vaccines could affect safety or effectiveness. Therefore, the CDC recommends that adults separate the COVID-19 vaccine from any others by at least two weeks.

If you’re getting a vaccine for the flu, shingles, pneumonia, or any other disease, be aware of the timing. You should not get any other vaccines within two weeks of any dose of a COVID-19 vaccine.

For more information about the COVID-19 vaccine, visit


National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases. Information about the Pfizer-BioNTech COVID-19 Vaccine. Centers for Disease Control and Prevention. Link

National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases. Information about the Moderna COVID-19 Vaccine. Centers for Disease Control and Prevention. Link

National Center for Immunization and Respiratory Diseases (NCIRD). Interim Clinical Considerations for Use of mRNA COVID-19 Vaccines Currently Authorized in the United States. Centers for Disease Control and Prevention. Link

National Center for Immunization and Respiratory Diseases (NCIRD). Recommended Child and Adolescent Immunization Schedule for ages 18 years or younger, United States, 2021. Centers for Disease Control and Prevention. Link

National Center for Immunization and Respiratory Diseases (NCIRD). Timing and Spacing of Immunobiologics. Centers for Disease Control and Prevention. Link

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