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A fourth dose of the Pfizer-BioNTech vaccine was insufficient to prevent infection with the Omicron variant of Covid-19, according to preliminary data from a trial in Israel.

Two weeks after the start of the trial of 154 medical personnel at the Sheba Medical Centre in Tel Aviv, researchers found the vaccine raised antibody levels. But that only offered a partial defence against Omicron, according to Gili Regev-Yochay, the trial’s lead researcher.

Vaccines which were more effective against previous variants offer less protection against Omicron, she said. Still, those infected in the trial had only slight symptoms or none at all. 

Last week, EU regulators warned that frequent Covid-19 booster shots could weaken the immune system and may not be feasible. 

Israel started rolling out the fourth dose of the vaccine to the over-60s and immunocompromised in late December amid a surge in cases. Since then, more than half a million Israelis have received the extra dose, according to the health ministry. 

The decision to give the fourth vaccine to the most vulnerable was the correct one, Regev-Yochay said at a virtual press conference, because it may have given additional benefit against Omicron. But the results did not support a wider rollout to the whole population, she said.

Researchers at Sheba are also conducting a trial using a fourth dose of the Moderna vaccine on 120 volunteers who had previously received three shots of the Pfizer-BioNTech jab. One week into the trial, they found a similar rise in antibodies to those that had been given four Pfizer doses, according to Regev-Yochay.

More stories like this are available on Bloomberg.com


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