Is 12 weeks between Covid jabs too long?

T he UK has a controversial policy of giving second doses of coronavirus vaccine 12 weeks after the first dose, rather than the three-week gap used in other countries. What is the reasoning behind what the Washington Post described as “one big, high-stakes science experiment”?

First, since there are limited doses of vaccine, this strategy means protecting more people, faster. This is an optimisation problem against supply constraints, time and a deadly virus. Modelling showed that, provided the first dose delivered reasonable efficacy, then most lives would be saved by lengthening the gap. By 17 February, there had been about 17 million vaccinations in the UK, with more than 16 million people having had a first dose. If a three-week gap had been used, only about 10 million would have had any vaccine.

Second, biology suggests that a delay should not be harmful, and may plausibly be beneficial. Andrew Pollard, the Oxford Vaccine Group head, said: “The immune system remembers the first dose and will respond, whether the later dose is at three weeks or three months.”

Third, the limited data available suggest that the great majority of protection comes via the first dose. The AstraZeneca vaccine trials had a range of intervals between the two doses, with a vaccine efficacy of 73% from 22 days up to 12 weeks after just one dose. The immune response was also larger following a longer interval. For the Pfizer vaccine, Public Health England estimated 91% efficacy for 15 to 28 days after the first jab alone, and a recent Israeli study found similar efficacy in their rollout. Although there is no direct evidence that this efficacy continues up to 12 weeks, there is no reason to think it would not.

In this global pandemic, both science and interventions have to be conducted at speed, using imperfect data and background knowledge to deal with inevitable uncertainty. And in a way the Washington Post was correct about an experiment: the UK is going to carry out a randomised controlled trial of different dose intervals, and this will be of huge benefit to the world.

• David Spiegelhalter is chair of the Winton Centre for Risk and Evidence Communication at Cambridge. Anthony Masters is statistical ambassador for the Royal Statistical Society