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.