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Britain will "look carefully" at claims that the Pfizer vaccine fails to protect as well as expected following research into the first 200,000 people given the jab in Israel, Sir Patrick Vallance has said.
The first real-world data showed the first dose led to a 33 per cent reduction in cases of coronavirus among people who were vaccinated between 14 and 21 days afterwards.
But that figure is far lower than that predicted by the joint committee on vaccines and immunisation (JCVI), which suggested a single dose would prevent 89 per cent of recipients from getting Covid-19 symptoms.
In a radio interview, Nachman Ash, Israel's vaccine tsar, said a single dose appeared "less effective than we had thought" and also lower than Pfizer had suggested, raising fears that giving only one dose will not be as protective as hoped.
Sir Patrick, the Government's chief scientific adviser, said experts would need to "keep measuring the numbers" but added that better immunity would build over time.
Speaking to Sky News, he said: "We need to look at this very carefully. What we know from clinical studies… is that if you take everything from day zero to day 28, then the overall figure is something like 50 per cent protection.
"But of course you don't expect any protection in the first days because your immune system hasn't had a chance to build up and some people may have been infected before they had the vaccine. If you take it from day 10 up to day 21 and beyond, it looks much more like the 89 per cent figure the JCVI gave."
However, Sir Patrick admitted the efficacy is unlikely to be as high as 89 per cent in practice because real-world rollouts of vaccines are often lower than trial results.
The Israeli data also showed that people who received their second dose of the Pfizer vaccine had a six to 12-fold increase, meaning they had far better protection.
The UK Government has been criticised for making people wait up to 12 weeks for a second dose, and even Pfizer has warned that one dose efficacy is around 52 per cent.
British scientists called for Israeli scientists to publish their data so they could check the results.
Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene and Tropical Medicine, said: "It is vital that advice and policy take into account the latest available data.
"However, the reports that have come 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 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 pre-print giving the detailed methods and data to understand and interpret these findings. 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."
Experts also warned that protection against picking up Covid may be less important than finding out whether it stops hospitalisation and deaths.
Although most trials were not powered to determine that, the early data suggests that some vaccines offer blanket protection against the kind of severe disease that leads to fatalities.