The UK will have to look "very carefully" at the effectiveness of a single dose of the Pfizer/BioNTech coronavirus vaccine, the country's chief scientific adviser has said.
Professor Patrick Vallance's comments came after reports from Israel suggested the protection it provides may be much lower than shown in trials.
Israelis began receiving first shots of the vaccine developed by Pfizer and its German partner BioNTech on 19 December last in one of the world's fastest vaccination roll outs.
Responding to Israel's claims that efficacy from the first dose of the Pfizer vaccine may be as low as 33%, Sir Patrick said studies showed that from day 10 after vaccination to 21 days and beyond, it was "much more like 89%".
However, he added that "when you get into real-world practice things are seldom quite as good as clinical trials".
He told told Sky News: "It 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."
According to reports, scientists in Israel - where around a quarter of the population has already been vaccinated - 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.
Professor Vallance said clinical study studies suggest that if you look at data from day zero, then the overall figure is around 50%.
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.
"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," he said.
Stephen Evans, professor of pharmacoepidemiology, London School of Hygiene and Tropical Medicine, said it was vital for advice and policy to take into account the latest available data.
He said: "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 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."
Israel is giving weekly data updates on its coronavirus outbreak to Pfizer under a collaboration agreement that may help other countries fine-tune their inoculation campaigns and achieve "herd immunity", officials said.
During weekly status reports, Israel will provide Pfizer with epidemiological data such as: the number of confirmed Covid-19 cases, hospitalisations, how many patients were on ventilators, how many died, as well as an age and other demographic breakdown. About a quarter of Israelis have received their first vaccine shot and 3.5% have already gotten their second dose.
The Israeli government has decided to extend the country's coronavirus lockdown to the end of the month after a spike in infections, despite the vaccination campaign.
Israel began its third lockdown in late December and tightened it on 8 January, with officials saying at the time it would be lifted after two weeks if the daily caseload decreased sufficiently.
Since the roll-out of vaccinations last month, more than 2.2 million of Israel's nine million inhabitants have been innoculated, Health Minister Yuli Edelstein said.
Despite the speedy operation, Israel said it had logged a daily record of a little more than 10,000 new cases in the preceeding 24 hours.
The rise in infections, despite the lockdown and vaccinations, has been attributed to virus mutations and disregard by some people of restrictions on assembly.
"The mutations... spread faster from person to person, and that expedites the epidemic," Dr Boaz Lev, head of the health ministry council advising on the vaccinations, told AFP.
"We know that mutations reached Israel from all kinds of places," he said, adding that rules closing schools were being flouted by some ultra-Orthodox Jewish institutions.
"Weddings, gatherings, ultra-Orthodox schools - all of these have an adverse effect in the morbidity increase, and caused this significant outbreak," Dr Lev said.
More than half a million cases have been reported and 4,005 people have died in Israel since the pandemic began.
Additional reporting AFP