I think the underlying issue is the same from both AZ and BioNtech

After the first doses of either AZ or BioNtech vaccine, the serum titer levels of antibodies and memory-T cells build up rapidly over a few days before levelling off. The second dose again causes a rapid increase in antibody levels before again levelling off - but at a higher level than before the 2nd dose.

In the stage 2 and 3 trials of the vaccines, a number of different dose and timing strategies were used. Not everyone in the trial got the exact same quantity of vaccine and not everyone got the second dose 21 days after the first. I believe some were 56 days after the first in the AZ trial.

The main findings were; i) that the longer you left the second dose, the higher the final antibody levels were. But not by much ii) the levels of protection granted by the first dose were sufficient to radically reduce the likelihood of hospitalisation and of death.

You can see that there is a bit of a trade-off to be made. The 2nd jab at 21 days gets a patient to - on average - the 90% protection level at day 28. A longer delay potentially gets the patient to a higher level of protection but they spend the time between the two jabs in a more vulnerable state.

The JCVI - an independent apolitical body - have the data and made the reccomendation. I think they looked at the number of fatalities expected for differing vaccination strategies based on estimates for vaccine supply and the rate at which the various age groups would receive the vaccines.

So if they followed the 21 day strategy they could get X million people to full protection and get Y deaths or follow the 12 weeks strategy and get 3X million people to an 'avoid death' level of protection and get 0.5Y deaths.

If a study suggests they can reduce the number of deaths markedly you can see why it would be adopted.

Posted By: Timmy_Goat, Jan 1, 19:52:22