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I have a family member who received their first dose of the Pfizer Covid vaccine but their second dose was scheduled for 4 weeks later. It was my understanding that the Pfizer doses were intended to be given 21 days apart as opposed to 28 days like Modernas version. In attempting to research the topic on my own, I am finding conflicting information from the CDC. On one hand, I am reading that the second dose can be administered up to 6 weeks later.... then, on the other hand, I found that there was only a 4-day grace period. With the understanding that guidelines change, what is the current advice on the timing of the second dose of the Pfizer vaccine? And if 21 days is ideal, does waiting longer diminish the effectiveness in any way? Thank you.
The normal interval for the two doses of the Pfizer vaccine is 21 days apart. However, due to problems with scheduling or incoming supply of vaccine, it is perfectly acceptable to have the second dose up to 6 weeks after the first dose. Also, because of possible scheduling problems we allow the two doses to be given within a 4-days window (i.e., 17 days for Pfizer) but do not suggest that it be given any earlier than the 4-day window AND this should only be for rare and extenuating circumstances. If there are significant delays in either the ability to be scheduled for an appointment or due to supply shortages, it is still okay if you receive the second dose after 6 weeks (i.e., it still counts and you will not have to repeat a 2 dose series).
The whole intent is that we have the two doses of the mRNA vaccine delivered within the schedule that was EUA approved, as closely as possible. Delaying the second dose does not harm the vaccine response but means a delay to the onset of “full protection.” However, there is a theoretical concern that giving the 2 doses too closely together in time can actually reduce the optimal vaccine response.
