Why two jabs?

[MrFoolish]

Where did the requirement for two jabs come from? I can't recall this being the case with other vaccines.

Did the initial trials test the effectiveness of one and two jabs?

[1nvest]

My layman understanding is that the first primes your immune system, but that 'memory' can fade relatively quickly, the second 'booster' more ingrains that into longer memory/protection.

Yes I believe that they investigated single/multiple doses and periods between to arrive at a optimal maximum protection choice. The UK opted to go beyond that 3 week/whatever optimal case (according to clinical trials) in order to get more vaccinated/protected sooner rather than later, for instance if the first provides 65% protection that is bolstered to 95% after the second dose then the preference was to have more with 65% protection sooner rather than fewer with 95% protection and many with no protection.

[9873210]

Where did the requirement for two jabs come from? I can't recall this being the case with other vaccines.

Did the initial trials test the effectiveness of one and two jabs?

There are many other vaccines that require two or more doses. Most of these are given to infants which is why they get about 1 shot a week for the first 6 months, and, perhaps, why you can't recall. However if given later they still need multiple jabs, for example if an adult has not been vaccinated for polio and requires adult vaccination they should get three shots at 1-2 and 6-12 month intervals.

[9873210]

My layman understanding is that the first primes your immune system, but that 'memory' can fade relatively quickly, the second 'booster' more ingrains that into longer memory/protection.

Yes I believe that they investigated single/multiple doses and periods between to arrive at a optimal maximum protection choice. The UK opted to go beyond that 3 week/whatever optimal case (according to clinical trials) in order to get more vaccinated/protected sooner rather than later, for instance if the first provides 65% protection that is bolstered to 95% after the second dose then the preference was to have more with 65% protection sooner rather than fewer with 95% protection and many with no protection.

All of the following is under an "as I understand it" caveat.

The phase III trials were not designed to arrive at the "optimal maximum protection" for an individual. They were designed to demonstrate adequate protection with high probability and reasonably quickness. A 3 or 4 week gap gets results 2 months faster than a 12 week gap. In the midst of a pandemic this was an important consideration. Similarly with the two shot series. We now know that a single Pfizer shot would have met the CDC requirement for 50% effective. There was discussion about that at the time, but it was felt that two jabs was more likely to demonstrate effectiveness. The J&J team faced the same decision but went the other way. We now have reason to believe that a J&J booster would increase effectiveness.

The initial phase III trials demonstrated effectiveness, not optimal effectiveness. There are ongoing efforts to research the effectiveness of different regimens. These may be useful to people and countries given the vaccine later, and it should not be surprising if there are changes to the doses or timing as we learn more and get closer to the optimum.

[GrahamPlatt]

We also now know that delaying the second dose a few weeks results in better immunity than giving it at the recommended interval.