Answering questions on COVID-19 vaccine second dose timing

CDC recommendation on second dose of vaccine recently updated

DETROIT – With the slow rollout of the COVID-19 vaccine Michigan residents are struggling not just to get their first shots, but for some getting that second dose in time can be an issue as well.

Given the difficulty people are having in making appointments, questions have been raised about the consistency of the vaccine supply.

Many people have been asking when they should get their second dose if they can’t get it exactly 21 days after for the Pfizer vaccine or 28 days after for the Moderna one.

The acceptable interval for the timing of the second vaccine dose has been a bit of a moving target as the CDC updates its recommendations to reflect certain realities about the vaccine rollout.

Michigan COVID-19 vaccinations: How to find appointments, info on phases


The current CDC recommendation updated late last week says, “second doses administered within a grace period of four days earlier than the recommended date for the second dose are still considered valid.”

That should clear things up for the many people who have asked about getting their dose a little earlier to accommodate other plans.

As for delayed doses the CDC’s updated guidance reads, “The second dose should be administered as close to the recommended interval as possible. However, if it is not feasible to adhere to the recommended interval, the second dose of Pfizer-BioNTech and Moderna COVID-19 vaccines may be scheduled for administration up to 6 weeks (42 days) after the first dose.”

There’s currently no available published data to support this very generous timeframe, but there’s similarly no reason to believe it’s not justifiable.

The bottom line is that the CDC’s current guidance should put many concerns to rest. The recommended intervals for the Pfizer and Moderna vaccines are based on the research that was done to secure the emergency use authorizations and that is what the published efficacy rates are linked to.


The fact is right now we don’t know the exact effect that significantly delaying the dose will have and everyone should do their best to get it on time.

There was early talk about only giving one dose to stretch the supply, and some people who had side effects from the first dose might think about skipping the second dose.

How important is the second dose?

Based on the studies that have been published it’s critical in order to reach a high level of protection.

There is some information to show that one dose provides a lower level of protection, but the exact amount is completely unclear.

That’s why in the absence of more information we’re sticking with a two dose requirement.

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