A preliminary study in Israel, believed to be the first in the world, has shown that the fourth vaccine against COVID-19 is not enough to prevent the omicron variant.

Gili Regev-Yohai, Director of the Infection Prevention and Control Unit at Sheba Medical Center, said the testing examined the effects of a booster dose of Pfizer after two weeks and a booster dose of Moderna after one week, Reuters reported.

The increase in antibodies after the fourth injection was "probably not enough for Omicron," she said.

The Sheba Medical Center conducted testing of a second booster vaccine among its 150 employees, and the results were preliminary and unpublished, The Times of Israel reported. About 500,000 Israelis had been vaccinated with the fourth dose as of Sunday.