A fourth dose of a Covid-19 vaccine increases antibodies in the blood fivefold after one week, Israel’s prime minister said, citing a preliminary study the day after the country began rolling out the jabs.

Israel began offering a fourth dose of the BioNTech/Pfizer vaccine on Monday to Israelis over the age of 60 and medical workers amid spiking case numbers due to the Omicron coronavirus variant.

Prime Minister Naftali Bennett said the findings of the study, by Israeli researchers, were promising.

“First of all, the fourth dose is safe. It’s a safety parameter similar to what we saw after the third dose — that’s good news,” Bennet said on Tuesday at Sheba Medical Center where the study was conducted.

“A week after taking the fourth dose we see a meaningful rise in the number of antibodies, a fivefold increase within one week,” he said. “That’s an indicator of a very high likelihood that the fourth dose will protect . . . against infection to some degree and against severe symptoms.”

 The study was carried out on 154 people from Sheba Medical Center who exhibited low antibodies and who received the third “booster” jab of the Pfizer vaccine before August 20. Other preliminary studies from Israel suggested that vaccine effectiveness wanes after three to four months.

 The European Medicines Agency signalled late last year that it was reviewing whether fourth doses were needed for a subset of at-risk populations, such as those who are immunocompromised. Immunocompromised people in the UK can receive a fourth dose.

 UK health officials have said they are reviewing the available evidence on whether this should be expanded to the general population.

Israel was the first country in the world to rollout a third shot last July, a move that was credited with halting the previous Delta variant wave. The fourth jab is now being offered to those who both meet the criteria and received the third shot more than fourth months ago. The government and health authorities have said the main aim of the fourth jab rollout is to shield the most vulnerable people from severe illness and death, avoid the collapse of the health system and keep the economy open. As of Monday, the first day it was on offer, about 100,000 eligible Israelis either received the vaccine shot or had already made appointments to do so.

 More than 10,000 new Covid cases were reported in Israel on Monday, the highest daily figure since September and a 10-fold increase from mid-December. The number of severely ill people has increased at a much more gradual pace and stood at 117 on Tuesday, of which 92 were not considered to be fully vaccinated according to health ministry data. Recommended Covid-19 vaccines Covid-19 vaccine tracker: Lowest-income countries lag further behind rest of the world While studies show that repeated boosting prevents infection, emerging evidence suggests double vaccination, without boosters, can still protect against severe disease, even when caused by Omicron. A Harvard university study this week found cellular immunity remained durable against the variant. The finding is one of the many pieces of evidence that will help global health authorities decide whether Omicron-targeted vaccines will become necessary.