A single shot of the BioNTech/Pfizer vaccine produces a robust antibody response within weeks, according to Israeli data that could help inform whether scarce global supplies can be stretched by delaying second doses.

At the Rambam Health Care Campus in northern Israel, 91 per cent of the 1,800 doctors and nurses that received the two dose vaccine showed a major presence of antibodies 21 days after their first shot, before receiving the second dose, according to Michael Halberthal, chief executive of the hospital. A further 2 per cent showed a moderate presence of antibodies.

“If 93 per cent had a major response three weeks after the first injection, this raises a good question, that you might rather be using the first injection on more people” said Dr Halberthal.

At the Sheba Medical Center, similar serological tests at different intervals showed at least 50 per cent of staff with a level of antibodies “above the cut-off point” two weeks after the first jab, said Arnon Afek, the associate director-general of the hospital chain.

The data from the two hospitals is based on individual antibody responses to the vaccine and does not a provide definitive assessment of the efficacy of a single shot. BioNTech/Pfizer’s clinical trials were based on two shots, 21 days apart, and did not measure antibody response. Pfizer said it could not comment on independent studies.

However, the early findings are likely to encourage those scientists who have argued that the time between the first and second doses of Covid vaccines could be extended in order to stretch limited supplies.

“This goes to what’s been in dispute right now in the UK, whether to continue giving just a first dose, so that people receive certain levels of immunity or to go to the second,” said Mr Afek,

Israel has sufficient vaccine supplies and is not expected to change its strategy. It plans to inoculate the entire adult population by mid March. “The question of whether the first shot is enough will not be answered out of Israel,” Mr Afek said. “Our policy is to give two shots, but the data we have collected is significant.”

Israel’s rapid vaccination programme is being closely watched by epidemiologists looking for data on real world vaccine efficacy and the impact of vaccine-induced immunity on infection rates. The country of 9m people has already administered first shots to more than a quarter of the population, the highest vaccination rate in the world. It has administered second shots to about 850,000 people, including 80 per cent of the population over 60.

Frontline healthcare workers were among the first to be vaccinated in Israel, providing a large sample of an at-risk population stretching back to late-December.

Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, said the UK, which has chosen to space the first and second doses of the BioNTech/Pfizer shot beyond the recommended 21 days, was conducting its own studies.

“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,” he said.