A man receives a fourth dose of coronavirus disease (COVID-19) vaccine as part of a trial in Israel, as Health Ministry is considering offering the second booster to the elderly and immunocompromised, at Sheba Medical Center in Ramat Gan, Israel December 27, 2021. REUTERS/Ronen Zvulun

Israel is to go ahead with second COVID-19 vaccine booster shots for people with weakened immune systems, the top government health official said on Thursday, but a final decision on wider usage is still pending.

An Israeli hospital administered fourth shots to a test group of health workers on Monday, in what it called the first major study into whether a second round of boosters will help contend with the Omicron variant. Results are expected within two weeks.

Israel was the fastest country to roll out initial vaccinations a year ago, and became one of the first to launch a booster program after observing that immunity waned over time.

Concerned about the rapid spread of Omicron, a Health Ministry expert panel last week recommended that Israel offer a fourth jab of the Pfizer /BioNTech vaccine to medical workers and those over 60 or with compromised immune systems.

But Health Ministry director-general Nachman Ash, whose approval is needed to launch a new booster campaign, did not immediately announce a decision, with some experts arguing that there was not enough scientific data to justify fourth shots.

At a news conference on Thursday, Ash said that he had decided that a second booster would be offered only to immunocompromised people for now and that he would continue to examine whether to broaden eligibility for the shot.

Cancer treatment and organ transplants, as well as chronic diseases, can weaken immune systems.

“In light of the gaps in knowledge in the world in the present situation, we are acting cautiously and responsibly,” Ash said.