The manufacturer of the Pfizer/BioNTech vaccine has said its efficacy has only been assessed for two doses given three weeks apart. Therefore the idea that a single dose will be protective beyond three weeks is speculative (Covid vaccine: chief medical officers defend rescheduling of second doses, 31 December). It would be truly tragic to vaccinate millions of recipients with the Pfizer/ BioNTech vaccine (at considerable effort and financial cost) with a twelve-week gap between doses if this doesn’t give them protection.
It is worth noting that there is likely to be a correlation between the antibody response and protection from infection. Therefore volunteers who have already completed two doses could be asked to give a small sample of blood to check the level of neutralising antibodies present four weeks from the first dose. Recipients whose second dose has been postponed after 4 January could give a similar sample from 11 January onwards to check their levels at the four-week point. A relatively small number of volunteers (perhaps 20 or 30 in each group) might settle this.