The Pfizer/BioNTech Covid vaccine is up to 85% effective after the first dose, a study has concluded.
The largest hospital in Israel, Sheba Medical Centre, looked at how effective the vaccine's first dose was among 7,214 of its staff.
Compared to those who were unvaccinated, there were much fewer staff who tested positive for coronavirus 15 to 28 days after the first jab.
Taking into account variants such as community exposure and the probability of Covid-19 contact, researchers found that all infections, including asymptomatic ones, were reduced by 75% 15 to 28 days after the first dose.
Even when vaccinated staff tested positive for Covid-19, they were less likely to develop symptoms, compared to unvaccinated staff.
Data showed there was an 85% reduction of symptomatic Covid-19-positive staff when they were vaccinated, compared to those who were not vaccinated.
The study, which has been peer reviewed, has been published in the Lancet medical journal.
Earlier this week, ITV News reported that a separate study by Israel's biggest healthcare provider Clalit found a 94% drop in symptomatic Covid-19 infections when people were vaccinated.
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Clalit researchers also discovered a 92% cut in severe illness from Covid-19 after two doses of the Pfizer/BioNtech vaccine.
In order to vaccinate as many people as possible, the UK government in December said the second dose would be given to people up to 12 weeks after the first dose.
The decision was based on the Joint Committee on Vaccination and Immunisation's advice. A report by the committee, updated in January, suggests that "short-term protection from dose 1 is very high from day 10 after vaccination".
The study by the Israeli hospital looked at vaccine efficacy up to four weeks after the first dose.
However, Professor Eyal Leshem, an infectious disease expert and director of Sheba's Institute for Travel and Tropical Medicine, told Sky News it is a fair assumption that efficacy would be maintained beyond four weeks.
He said the data suggests "the further we go from the first dose, the better protection is".