system
Given the following post, respond as if you're commenting on Reddit:
human
Vaccine Efficacy After Manufacturer Recommended Booster Period?

 My FIL brought up a good point as we were discussing the vaccine I received. Our provincial government has mandated that we push back booster shots well beyond the recommended period, in this case, it's four months from twenty-one days for Pfizer. He said that the manufacturer has set twenty-one days for a reason, so how effective is the booster after four months. His position is that it's pointless to even get it if the booster shot is worthless after even twenty-two days.
gpt
You gotta realize that the "manufacturer recommended" period is literally the **only** one they tested to get the vaccine approved. There is no evidence its the optimal one, in fact its very unlikely to be as the optimal period is much longer for practically all vaccines.

Why did they pick 21 days? They had to start somewhere, they were in a hurry to get the vaccine approved and 21 days was about the shortest period that was worth trying.