According to a preliminary study in Israel, a fourth dose of COVID-19 vaccination raises antibodies to even greater levels than the third dose, but it is insufficient to prevent Omicron infections.

According to Gili Regev-Yochay, director of the Infectious Diseases Unit at Israel’s Sheba Medical Center, the Pfizer booster was given to 154 participants after two weeks and the Moderna booster to 120 people after one week in a trial.

These were compared to a control group that didn’t get the fourth injection. According to the hospital, those in the Moderna group had already received three doses of Pfizer’s vaccine.

The vaccines led to a increase in the number of antibodies “even a little bit higher than what we had after the third dose”, said Regev-Yochay.

“Yet, this is probably not enough for the Omicron,” she told reporters. “We know by now that the level of antibodies needed to protect and not to got infected from Omicron is probably too high for the vaccine, even if it’s a good vaccine.”

The findings, which the hospital said were the first of its kind in the world, were preliminary and not yet published.

Israel was the fastest country to roll out initial vaccinations against COVID-19 a year ago and last month started offering a fourth shot, or a second booster, to the most vulnerable and high-risk groups.