Is the British approach of prioritising first doses of Covid vaccines and not promising a second dose until 12 weeks later compromising our ability to fight the disease? It is not a moot point, with several EU figures asserting that it is a risky route to take.
As I wrote here a couple of weeks ago, as far as the AstraZeneca/Oxford vaccine is concerned, what evidence we have supports the practice of delaying a second dose until 12 weeks after the first one; the vaccine is more effective that way.
However, a question mark has continued to hang over the Pfizer/BioNTech vaccine. There is limited data on the most effective dosing regime in this case because the phase 3 trials did not experiment with delayed doses – everyone involved was given a second dose 21 days after the first.
An Israeli study published in the Lancet, though, does now give us a better idea of the efficacy of the Pfizer vaccine after just one dose. The researchers administered first doses to 7,000 healthcare workers at the Sheba Medical Centre starting on 19 December, when Covid infections were raging in Israel. Within 28 days there were 170 cases of infection along the workers, of which 99 were symptomatic. Between days 1 to 14 after vaccination there were 5.5 infections per 1,000 person-days, falling to 3.0 between days 15 and 28. As for symptomatic infections, the rates were 2.8 and 1.2 respectively. Comparing this with unvaccinated healthcare workers in Israel, a single dose of the Pfizer vaccine was found to reduce all infections by 30 per cent between days 1 and 14, and 75 per cent between days 15 and 28. For symptomatic infections the efficacy rate was 47 per cent between days 1 and 14 and 85 per cent between days 15 and 28. By comparison, real-world data from Israel has found that the Pfizer vaccine has an efficacy rate of 94 per cent after two doses.
The study should provide some reassurance for the UK approach, although the findings do need to be treated with some caution. We still don’t have data on the efficacy of a single dose of the Pfizer vaccine beyond 28 days. Given that the UK approach is to wait up to 84 days before administering a second dose this is quite a gap. Moreover, the participants in the Israeli study were all at a single hospital, and were compared with healthcare workers elsewhere. We can’t assume that the Sheba Medical Centre would have suffered the same infection rate as elsewhere over the period studied.
However, PHE data reported in the Telegraph this morning (which will not be published until later this month) does seem to suggest that single doses of either the Pfizer or AstraZeneca vaccine are helping to reduce infections by two-thirds – which may provide some justification for the UK approach of delivering single doses to as many people as possible in the first instance.