A single dose of the vaccine from Pfizer Inc. and BioNTech SE reduced COVID-19 infections by 85 per cent in a study in Israel, bolstering the U.K.’s decision to speed immunizations by delaying a second shot.

Among health-care workers who got the vaccine, symptomatic infections were reduced by that percentage in the 15 to 28 days after the first dose, compared with those who didn’t get a shot, according to the report in The Lancet medical journal. While most workers received a second dose on schedule -- about three weeks after the first -- the booster would only have just started to kick in by the end of the study, so it was essentially looking at the effects of one dose, researchers said Thursday.

Israeli researchers analyzed the real-world effects of the Pfizer shot among staff at Sheba Medical Center, the country’s largest hospital. Although it wasn’t a standard clinical trial like those used to establish the efficacy of drugs and vaccines, it gives preliminary support to health officials who recommend postponing second shots to quickly get first doses to as many people as possible.

Countries delaying second doses need to “understand the significance of the decision to give a single shot, and we show it carries quite substantial protection,” said Arnon Afek, Sheba Medical Center’s associate director general and a co-author of the study.

The U.K. now allows as many as 12 weeks between shots. After rejecting a similar dose-stretching policy, U.S. public health advisers are weighing whether to extend the interval between first and second doses, Bloomberg News reported.

“Early reductions of COVID-19 rates provide support of delaying the second dose in countries facing vaccine shortages and scarce resources,” the researchers concluded in the Lancet analysis.

In further support for Britain’s immunization program, vaccinations appear to be cutting infections and transmission by about two-thirds, the Telegraph reported, citing government sources. The U.K. has used the Pfizer vaccine alongside a shot from AstraZeneca Plc and the University of Oxford that’s also delivered in two doses. The vast majority of recipients have gotten only a first shot so far.

The Israeli researchers retrospectively analyzed COVID-19 infections among more than 9,000 health-care workers who were eligible for the vaccine. The data was adjusted to reflect differing Covid exposure rates in vaccinated and non-vaccinated groups.

Case numbers were unlikely to have been affected by the second dose of vaccine, because it takes about a week to kick in, by which time the study period was over, said Sharon Amit, another study co-author and director of clinical microbiology at Sheba Medical Center. Also, it usually takes about a week after being exposed for most people to show symptoms, so anyone who became sick toward the end of the 28-day period was likely to have been infected before their second dose.

“Follow-up to assess long-term effectiveness of a single dose is needed to inform a second dose delay policy,” the researchers cautioned in their report.