Israel's coronavirus chief warned the first Pfizer vaccine dose seems "less effective" than expected.
This may worry the UK and US, which are prioritizing widespread first doses.
Israel has vaccinated a larger share of its population than any other country.
The Israeli official leading the country's coronavirus response warned that it is seeing a smaller effect than it hoped after giving a dose of the vaccine.
The nation has had the world's fastest vaccine rollout, and as of January 19 had given a first dose of the Pfizer/BioNTech vaccine to 25.6% of its population, per Our World in Data.
But a note of caution came from Nachman Ash, Israel's coronavirus commissioner, who told Israel's Army Radio that a single dose appeared to be "less effective than we had thought," according to The Guardian.
The vaccine is designed to come in two shots, with the second dose given three weeks after the first in clinical trials. This is how Israel is distributing the vaccine.
But the strategy raises concerns for the UK, which is prioritizing giving people the first dose of the vaccine.Â
This means delaying second doses by as much as 12 weeks so that as many people as possible can get their first dose one. The hope is that partial immunity among many people is better than fuller immunity for fewer people.
And it also could bring concerns for the US, where incoming president Joe Biden plans to release all available vaccine doses to maximise the number of people getting shots, which could result in delays to second doses even though the US plan is to give them all on schedule.
Pfizer says that a single dose of its vaccine is about 52% effective, while getting a second dose makes it around 95% effective.
According to Israel, the single dose appears to only be around 33% effective, a significant loss.
Sir Patrick Vallance, the UK government's chief scientific adviser, told the UK's Sky News that he will look "very carefully" at the level of protection that people are getting.
He did not say that the UK should change its strategy, but that the government would "just need to keep measuring the numbers" as the vaccine is given to people.
UK scientists had said in December that clinical trial data suggested the Pfizer vaccine would be 89% effective around 10 days after one dose.
Vallance said this week that the real-world rate of effectiveness was always expected to be lower than that, but that he doesn't think it will be "as low" as what Israel has reported.
The first dose of the vaccine is not thought to offer any protection until around 10 days after getting the shot, and including those days when trying to figure out how effective the first dose is would drive the numbers down.
It is important to note that the UK is not only using the Pfizer/BioNTech vaccine. It is also using the AstraZeneca/Oxford University vaccine, where studies suggest that a spacing out doses could actually provide more protection.
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