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Do You Have To Get Your Second COVID-19 Shot Exactly 21 Or 28 Days After Your First?

A vial of the Pfizer-BioNTech vaccine for COVID-19.
Gabriel C. Pérez
Pfizer-BioNTech's COVID-19 vaccine requires a second dose at least three weeks after the first for full protection from the virus.

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Updated Feb. 10

QUESTION: Is it necessary to get the second shot exactly 21 or 28 days later, or is it at least that amount? What if something comes up and you can't make it for the second shot on exactly that day? — Susan

ANSWER: No, you do not need to get the second shot exactly 21 or 28 days after the first. The Centers for Disease Control and Prevention says you can get the second dose later than the recommended interval, but you shouldn’t get it sooner.

The two COVID-19 vaccines approved in the U.S. right now require two doses for full protection. Pfizer-BioNTech’s vaccine calls for a second dose 21 days after the first, and Moderna’s calls for the second dose 28 days later.

If you can’t schedule an appointment exactly 21 or 28 days from your first appointment, that's OK. The vaccines can be administered up to 42 days after the first, according to the CDC. Beyond that window, there's not a lot of data available on the vaccine's efficacy.

The agency does urge people to get the second shot as close to those 21- and 28-day timeframes as possible so they can get full protection as soon as possible.

Some people have gotten their second dose earlier than the recommended timeframe. The CDC says: "Second doses administered within a grace period of 4 days earlier than the recommended date for the second dose are still considered valid. Doses inadvertently administered earlier than the grace period do not need to be repeated."

After getting the second dose, health experts warn, you won’t immediately be fully immune. It takes time for your body to build up immunity, and the vaccine may not protect you until one or two weeks after the second shot, the CDC says.

You don’t have to go to the same provider you went to the first time to get your second shot, says the Texas Department of State Health Services, but your provider should receive a second dose for you. If for some reason you don’t go to the same provider, make sure your second shot is from the same manufacturer as the first. Your provider should give you information about what vaccine you got and when you should come back for a second dose, DSHS says.

This post has been updated to include information about second doses received before the 21 or 28 days. It has also been updated with information on the 42-day window.

Marisa Charpentier is KUT's assistant digital editor. Got a tip? Email her at Follow her on Twitter @marisacharp.
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