A fourth shot may not offer much more protection against catching Omicron, an early Israeli study suggests.

JERUSALEM — Preliminary results from a small Israeli study examining the effects of a fourth coronavirus vaccine dose suggest that despite an increase in antibody levels, the additional dose a few months after a third shot may not do much more to prevent infection with the Omicron variant.

Prof. Gili Regev-Yochay, the director of the Infectious Disease Unit at Sheba Medical Center and the leader of the research, cautioned that the results were not yet peer-reviewed. But she said that there was no evidence of any risk from getting a fourth dose, and that she supported giving the shots to vulnerable people who might have some benefit from it.

She said the study’s initial results raised questions about giving the extra dose more broadly, including to generally healthy people in their early 60s. For them, she said, it might be preferable to wait for a newer vaccine adapted to deal with variants like Omicron.

On Tuesday, Dr. Sharon Alroy-Preis, a senior official at Israel’s Health Ministry, told Kan public radio that the decision to offer a fourth dose to older and more vulnerable people had been correct, since they had produced higher levels of antibodies. Higher levels of antibodies likely would give added protection against serious illness.

Initial results from the study showed that the fourth dose boosted antibodies fivefold. But Professor Regev-Yochay told reporters that two weeks after 154 hospital workers received a fourth dose of the Pfizer-BioNTech vaccine, the number of participants infected with the coronavirus was only slightly lower than the number of infected in a control group that had not received a fourth shot. The hospital did not release the raw data.

The study adds information to the debate over whether a fourth shot is advisable. Israel made the shots available to Israelis aged 60 and over and to people who are considered particularly vulnerable before much data was available. The Health Ministry acknowledged a lack of a scientific basis at that time, but said that the rising rates of new cases meant time had run out for deliberating and gathering more data.

Israeli data shows that immunity provided by a booster shot begins to wane after a few months, and officials said that any downsides of getting a fourth shot were outweighed by the possibility that it could add protection as Omicron spread.

Among scientists, questions remain over whether fourth shots are advisable. Research indicates that Covid vaccines already protect against the worst outcomes, including from the Omicron variant. And while initial boosters in general are recommended in several countries, doses every few months may not be a viable long-term strategy, some experts have said.

About half a million Israelis have received a fourth dose.

Among some Israeli professionals, the rush to give a little-tested fourth dose was a bone of contention from the start.

An advisory panel of experts recommended administering it to people aged 60 and over, as well as to people with weakened immune systems, on Dec. 21, and Prime Minister Naftali Bennett enthusiastically supported the proposal. But given the lack of knowledge about Omicron then or the effectiveness of a fourth dose, some experts called the idea premature.

Israel’s Ministry of Health approved rolling out the fourth vaccine for those 60 and older in early January.

It was not immediately clear whether the news from Sheba would slow the uptake among people eligible to receive a fourth dose.

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