Israel approves a 4th dose of Covid vaccine for those with compromised immune systems.

JERUSALEM — Israel on Thursday approved a fourth dose of Covid vaccine for people with weakened immune systems, moving ahead of much of the world in taking a little-studied step to try to protect its most vulnerable against the fast-spreading Omicron variant.

The decision by health officials came more than a week after an advisory panel of Israeli experts recommended a fourth dose for people with weak immune systems, as well as for others at high risk, including those aged 60 and over and health workers.

The panel had acknowledged the uncertainties and lack of data surrounding both Omicron and the fourth dose, but presented data indicating a significant waning of immunity in people aged 60 or older, who were the first to receive their third shots starting in August.

At a news conference on Thursday evening, Prof. Nachman Ash, the director-general of Israel’s Ministry of Health, said the advisory panel’s recommendation had been partially accepted, and a fourth dose would be offered to those whose immune systems have been compromised by illness or treatments they are undergoing. “Given the gaps of knowledge that exist around the world regarding the efficiency of a fourth dose in the current circumstances, we are acting cautiously and responsibly,” Professor Ash said.

He said Israeli health officials would continue to monitor the effects of Omicron in Israel and abroad, including the risk of severe illness in people who are already vaccinated, before taking any decision to broaden the rollout of a fourth dose for other vulnerable people, including the older population.

Weighing the potential benefits of another booster shot against the risks, some scientists, including a few on the Israeli advisory panel, have voiced concern that too many vaccinations might cause a sort of immune system fatigue, compromising the body’s ability to fight the coronavirus, particularly among older people.

The Sheba Medical Center, near Tel Aviv, began a study on Monday to test the safety and effectiveness of a fourth dose of a vaccine, administering an additional shot to 150 medical personnel who had received a third dose of the Pfizer-BioNTech vaccine at least four months ago. After Thursday’s announcement, officials there said the hospital would start administering fourth doses to heart transplant patients on Friday morning.

Developments in Israel, an early leader in Covid vaccinations, are being closely watched as governments worldwide struggle to confront the rapidly spreading Omicron variant, which is driving record numbers of new infections in parts of the United States, Europe and many other places. Israel was the first country to roll out third shots as boosters for the population at large, putting it in position to assess earlier than other countries how effective the shots are and how quickly the protection might wear off.

Even as some studies suggest that Omicron infections are milder than those caused by other variants, the surges are already stretching health systems. Israel’s hospitals have already filled up with patients suffering from complications of winter flu and other respiratory ailments.

At least half the new daily cases in Israel are now believed to be from Omicron, and experts say that variant is likely to surpass Delta as the dominant variant in the country within a couple of weeks.

Over four million Israelis have received a booster shot, out of a total population of nine million, but about a million eligible Israelis have not. In November, Israel became one of the first countries after the United States to approve Covid vaccinations for children aged 5 to 11, but the initial uptake in that age group has been slow.

On Thursday, Israel received its first shipment of Pfizer’s coronavirus drug. Known as Paxlovid, it was approved by the U.S. Food and Drug Administration last week for Covid patients age 12 and over who are vulnerable to becoming severely ill because of their age or prior medical conditions.