A fourth shot of the coronavirus vaccine does not provide much additional protection against Omicron, according to an Israeli study.
"The rise in antibody levels that we saw with both Moderna and Pfizer are slightly higher than what we saw after the third booster vaccine," said Gili Regev-Yochay, director of the infectious disease unit at Tel Aviv's Sheba Medical Center, who led the research.
However, "despite increased antibody levels, the fourth vaccine only offers a partial defence against the virus," she added.
Regev-Yochay said that breakthrough infections were also being observed in people who had been vaccinated four times.
She emphasized that the results were preliminary and therefore did not give more precise figures.
Around 150 participants in the study received a fourth dose of the Pfizer-BioNTech vaccine two weeks ago. A week ago, another 120 participants received a fourth vaccination with Moderna after three doses of Pfizer-BioNTech.
This is the first trial worldwide studying a fourth vaccination with combined vaccines, Regev said. The results of both groups after one week were very similar, she said.
Israel's decision to give a fourth dose to immunocompromised people might have a small advantage, according to Regev-Yochay. However, she did not recommended extending this offer to the entire population.
People over 60 and medical personnel are also among those currently entitled to a fourth vaccination in Israel.
Regev spoke of a dilemma over whether to continue giving the fourth dose to healthy people over 60, given the early results of the study.