TEL AVIV — Israel’s decision this week to become the first country to recommend a fourth vaccine dose to combat the highly contagious omicron variant came after health officials concluded that an initial booster had turned the tide this fall against the delta variant.

While they acknowledged that their decision was not based on new scientific data about the omicron variant, officials said they thought it would be prudent to recommend an additional shot because they believe that the ability of the initial booster to prevent infection has been waning over time.

The decision, announced by Israeli officials Tuesday, will make a fourth dose — or second booster — of the Pfizer-BioNTech vaccine available to people over 60, those with compromised immune systems and employees in the health-care sector. All eligible recipients would need to have had their third dose more than four months prior.

The decision is still awaiting confirmation by Health Ministry Director Nachman Ash before becoming national policy. But facilities across the country are preparing to begin administering the vaccine, with many saying they are ready to start as early as Sunday.

Israel’s national coronavirus advisory committee made its recommendation for the fourth dose Tuesday while still gathering data on the omicron variant, saying it does not have the luxury of time. The omicron variant is believed to be three times more transmissible than previous variants, and while it may be milder — it has not caused massive increases in hospitalizations in the United Kingdom, and infections have plummeted after a surge in South Africa — many Israeli health officials warn that an increase in even moderate cases could overwhelm the country’s hospitals.

According to Health Ministry data, 1,400 people tested positive Wednesday for the coronavirus, 45 percent of whom were fully vaccinated. In Israel, that means they were at least 12 years old and had received a booster shot at least one week prior, or were within six months of having received a second vaccination shot.

“It’s a risk-management issue,” said Arnon Afek, deputy director of Sheba Medical Center in central Israel, who is a member of the policy section of the national coronavirus advisory committee.

“The committee looked at the incredibly fast spread of omicron, in the U.K. and other countries, and decided that we do not have the time. They decided that we should work in parallel: First give the recommendation to immunize, and then run the studies,” he said.

When Israel launched a campaign over the summer for booster shots in response to the delta variant, it based the decision on studies from Israel and abroad showing that vaccine efficacy significantly waned in at-risk people at least five months after they received the second dose.

Experts mostly agree that the boosters — first offered to people over 60 and vulnerable groups in July, then to most of the general population in August — helped crush the delta variant wave in Israel, allowing the economy and schools to remain open. They credited the vaccine — more than social distancing enforcement, closing the borders or other measures that helped only to delay infection — with making the strategy successful.

In the weeks that followed, Clalit, Israel’s largest health provider, found that a third shot of the Pfizer-BioNTech vaccine was 92 percent effective in preventing serious illness and 93 percent effective in preventing hospitalization, compared with decreased protection from only two doses, according to a large-scale study published in the Lancet medical journal in October.

Yasmin Maor, director of the infectious-disease unit at Wolfson Hospital and a member of the advisory committee, said in a news conference Wednesday night that it was “unfortunate” that the booster seems to provide insufficient protection against the omicron variant during Israel’s “fifth wave.”

“We would have been happy to have a perfect product, but we have to note how many lives were saved because we administered a third vaccine dose,” she said.

“When the booster was new, there was little spread of infection. The farther we move away from it, we are seeing double the rate of infection,” Boaz Lev, who heads the advisory committee, told reporters. “The price will be higher if we don’t vaccinate.”

Israel has confirmed at least 341 omicron variant cases, most of which were traced to travelers who returned to Israel from abroad.

On Monday, Israel added 10 countries, including the United States and Canada, to its “red list” of banned destinations, which also include several European countries and almost all of Africa. The entry of noncitizens has been banned since last month.

Amnon Lahad, chairman of family medicine at Hebrew University, said Israel’s fourth-dose campaign was based on “panic,” rather than science.

Instead of going forward with the next round of vaccines, he said, Israel should dedicate resources to inoculating the country’s estimated 1 million unvaccinated people, many of whom are members of the country’s underserved populations and lack easy access to a vaccination facility.

Out of a population of 9.3 million, 6.4 million have received their first shot, 5.8 million their second and 4.1 million their third, according to the Health Ministry.

The towns with the lowest rates are ultra-Orthodox, Arab and lower-income communities.

Israel has recorded more than 8,200 coronavirus-related deaths. It has not confirmed any deaths from the omicron variant.

On Wednesday, the Health Ministry said that a 65-year-old man who died the day before had contracted the delta variant, not the omicron variant as originally reported.

Lahad criticized Israel’s policy of classifying documentation from cabinet meetings and other government discussions about coronavirus policy, saying that the “secrecy” was contributing to vaccine hesitancy across the country.

“Israel is behaving like it’s in a war,” he said.