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 released Monday.

Two weeks after the start of the trial of 154 medical personnel at the Sheba Medical Center in Tel Aviv, researchers found the vaccine successfully raised antibody levels.

But that only offered a partial defense against omicron, according to Gili Regev-Yochay, the trial’s lead researcher. Vaccines which were more effective against previous variants offer less protection with omicron, she said. Still, those infected in the trial had only slight symptoms or none at all.

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, since it may have given additional benefit against omicron. But she added the results didn’t support a wider rollout to the whole population.

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.