The UK’s chief scientific adviser has admitted experts will have to look "very carefully" at the effectiveness of a single dose of the Pfizer/BioNTech coronavirus vaccine after data from Israel's rollout brought Britain's strategy into question.
Sir Patrick Vallance’s comments came after reports from Israel suggested the protection one dose of the jab offers may be much lower than figures used to justify Britain's strategy of delaying time between first and second doses.
Initial data on the vaccine led UK experts to believe patients were given 89% protection from day 10 after vaccination to 21 days and beyond, but Israel claims efficacy from the Pfizer vaccine may be as low as 33%.
When Sky News put figures from Israel to Sir Patrick, he responded: "[Efficacy] probably won't be as high as that in practice, but I don't think it'll be as low as the figures you've just given.
"When you get into real-world practice things are seldom quite as good as clinical trials," he said.
Israel, where around one-quarter of the population has already been vaccinated, is one of the only countries in the world with enough data to make a proper assessment of the figures.
According to reports, scientists there studied preliminary data from 200,000 vaccinated people.
The study reportedly suggests that a drop of 33% in positivity was seen in the vaccinated group on day 14 after vaccination.
Sir Patrick said clinical study studies suggest that if you look at data from day zero, then the overall figure is around 50%.
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But he added that protection is not expected in the first 10 days because the immune system has not had the chance to build up and some people may have been infected before they had the vaccine.
Sir Patrick said: "I don't know exactly what Israel are looking at - they're looking at the total period from day nought and that doesn't give an exact comparison.
"But we need to look at this very carefully - we just need to keep measuring and understanding it."
There is controversy around the UK's vaccination strategy after the chief medical officers recommended a delay of 12 weeks in between jabs - rather than the three advised by the World Health Organization - so first doses can be given to more people in a shorter space of time, rather than fewer people receiving both doses.
Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, said reports from Israel are "insufficient to provide any evidence that the current UK policy in regard to delaying the second dose of vaccines is in any way incorrect".
"The reported efficacy of one dose has not been compared using the same methods and patients with the efficacy of two doses at 84 days.
"The details of the different studies have only been released, it seems, at a press conference the reasons for which are unclear.
"There is a need for at least a preprint giving the detailed methods and data to understand and interpret these findings."
He added: "It is not sensible to compare efficacy derived from an observational study of this type which is subject to many biases, with the efficacy derived from randomised trials."
Prof Evans explained that it was important to recognise that in the UK, efficacy against detected Covid-19 may be less important than efficacy against hospitalisation and death.
He said: "The UK will soon have its own data showing efficacy after the first dose for the different vaccines currently in use and any policy changes should await more robust data.
"If, for example, the efficacy after one dose was 33% but the efficacy after two doses was 60% the UK policy would still be justified.
"Similar arguments apply in relation to efficacy against serious disease."
A Downing Street spokesperson said: "As we set out at the time when we said we would be moving the first vaccine dose followed by another in 12 weeks, it remains our priority to protect as many people as possible from the virus.
"That is why we took that decision.
"That decision was based on a review of the data available, which showed that the Pfizer vaccine was 89% effective in protection against Covid between 15 to 21 days after the first dose.
"It remains the case that the chief medical officer and the JCVI recommend offering one dose first and another dose up to 12 weeks after. "