LONDON - The British government will look "very carefully" at concerns the Pfizer-BioNTech coronavirus vaccine's initial effectiveness is low, the UK's top government scientist has said.

Israel, which has already given about a quarter of its population a first dose, has warned that it could be as low as 33% effective after the first injection.

That may have implications for the UK's vaccine strategy, which focuses on giving as many people an initial dose before providing the booster jab up to 12 weeks after.

Speaking to Sky News, Sir Patrick Vallance, the government's chief scientific adviser, said between day 10 after the first dose and 21 days after, it was "much more like 89%" efficacy in studies.

However, he warned that "when you get into real-world practice things are seldom quite as good as clinical trials".

"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," he told Sky's Q&A.

"We need to look at this very carefully, we just need to keep measuring and understanding."

Reuters reports that Israel is working with Pfizer to provide data on the vaccine.

Sir Patrick said more data will become available in the coming weeks and stressed vaccines will protect recipients from severe illness and reduce the chance of dying.

Although less is known about how much it will reduce transmission of Covid-19, he said the government thinks it will have some effect but does not know how much.

Sir Patrick also warned that even if care home residents had received both doses of a vaccine, it was not yet safe to visit relatives there.

"This is a horrible situation, and many of us have got relatives who are in care homes and how difficult it is not to be able to see them and not to be able to do the things that we all wish to do – the normal human things – but I'm afraid my answer to this question is no, it's not safe," he said.