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Within days of the midnight vote that decided to distribute a third shot, the first of millions of booster shots were administered, months before the U.S. or any other country would take the same step.

“It was a really tough discussion,” said epidemiologist Gili Regev-Yochay, who presented key research on the effectiveness of booster shots. “[But] it was a decision that was reached essentially with one voice.”

Throughout the global effort against Covid-19, Israel’s public health experts have been consistently ahead of their counterparts elsewhere in the world. By securing an early supply deal with Pfizer for its vaccine, sweetened in part by a promise to share data from Israel’s extensive network of health maintenance organizations, they have had an edge in understanding how the vaccine behaves in the real world.

Shoppers pass between stalls in Carmel Market in Tel Aviv last week.

Photo: Kobi Wolf/Bloomberg News

They were also willing to act quickly when they saw that data, among other things suggesting to extend the vaccine to younger teenagers and to adopt a Covid passport system to persuade holdouts to get their shots if they wish to visit cinemas, restaurants or other entertainment sites.

At times the country has moved faster than some other nations might prefer. The World Health Organization, among others, has been critical of vaccinating younger teenagers or providing third booster doses at a time when billions of people in poorer countries haven’t yet had their first shots.

But where Israel goes, others often follow. President Biden received a booster shot at the White House on Monday, days after the U.S. Food and Drug Administration cleared it for people aged 65 and older and adults in high-risk groups, a recommendation backed by the Centers for Disease Control and Prevention. The U.K, which was also quick to roll out vaccines, started this month to offer booster shots to over-50s and vulnerable people, while the European Union regulator will decide whether to endorse a third shot early next month.

Those involved in the decision-making process in Israel credit a culture of debate and a willingness to improvise, instilled in part by long careers in the military common among senior health professionals, and tested during national security crises.

Israeli Prime Minister Naftali Bennett accompanies his mother Myrna Bennett as she receives her third Covid-19 vaccine shot last month.

Photo: Press Pool

Arnon Afek, a doctor and member of the advisory group, served as a senior officer in the medical corps and later as Health Ministry director-general before becoming director of the Sheba Hospital in Tel Aviv. When the pandemic first struck, he said he turned part of his hospital’s parking garage into a coronavirus ward.

“We are always living on the verge of an emergency,” Dr. Afek said. “It might be from the Gaza strip or Covid or cyberwars against our enemies. We know how to rely on ourselves and know how to deal with emergency situations.”

Since members of the advisory panel all work pro bono, neither politicians nor government officials can stop them speaking their minds when they meet with Israel’s cabinet.

“Nobody has any leverage on me when I sit in that room,” said Ran Balicer, head of the primary experts advisory committee on Covid-19, who said he does most of his advisory work at night after his busy day job as a senior official at Israel’s largest health care organization Clalit.

Ran Balicer, chief innovation officer for Healthcare Maintenance Organization Clalit, in Ramat Gan, Israel, in February.

Photo: rami amichay/Reuters

Israel’s leaders have largely listened to the scientists, the advisers say, despite a protracted period of political turmoil that saw Prime Minister Naftali Bennett dislodge long time leader Benjamin Netanyahu. One of the few times when the government has diverged from the advice it was given was when Mr. Netanyahu wavered last year on creating a tiered system of lockdowns like a traffic light: red, green and amber. Israel later had to impose another nationwide lockdown.

The medical professionals who voted to approve boosters shots—perhaps the most contentious issue they have faced—were swayed by two studies.

One was a Health Ministry analysis of Israeli data that showed those vaccinated in January through March were far more likely to get sick than those inoculated later.

The second study to swing the late-night debate behind a shot for over 60s was presented by Dr. Regev-Yochay and her team at Sheba Hospital. It pointed to the sharp rise in infections coming not only from the virulence of the Delta variant, but also from the waning prevalence of antibodies in people who had been vaccinated.

Her research showed that resistance to the virus had been falling in test subjects for months. They also knew from booster shots given to immunocompromised patients at Sheba that a third dose could successfully increase antibodies. The rest of the panel agreed to proceed.

Pfizer said at the same time that its own research also showed that antibody levels were waning over time.

Students return to school after the summer break, less than a month into a Covid-19 vaccine booster drive, at Arazim Elementary School in Tel Aviv at the beginning of September.

Photo: amir cohen/Reuters

The decision quickly paid off. When the booster shot campaign began, the majority of Delta infections were detected among patients who had already been vaccinated and had assumed they were safe from the virus. Infection levels per capita were among the highest in the world and it appeared the country was headed toward another economically debilitating lockdown.

Since then, more than three million of Israel’s nine million inhabitants have gotten a booster shot, including a majority of those in at-risk groups. Most infections are now among those who are unvaccinated, and talk of a fourth lockdown has been replaced with discussions about learning to live with the virus.

There is a small minority of Israelis who think the country is moving too quickly and resent being at the forefront of how to deal with Covid. Eldad Yaniv, a prominent political activist and critic of Mr. Netanyahu, has now taken Mr. Bennett’s government to task for moving more quickly than the U.S. Food and Drug Administration in approving boosters.

“An experiment is taking place in Israel,” he wrote in a Facebook post on Sept. 12, arguing Israel’s deal with Pfizer for vaccines is propelling it to push ahead of other countries in approving third shots for the new mRNA vaccines.

Israeli officials say their deal with Pfizer is not exclusive, nor obliges them to expand their vaccination campaign quicker than other countries.

Most of Israel’s doctors and epidemiologists remain undeterred, turning their firsthand view of the vaccines’ effects into research that much of the world has turned to. Dr. Balicer was the lead writer on a landmark paper published in the peer-reviewed New England Journal of Medicine in February, showing that Pfizer was effective in real-world conditions. He later led another paper in the same journal that demonstrated that the risks of getting ill from Covid-19 outweigh potential risks from receiving the vaccine.

While the FDA was deliberating whether to approve boosters last week, an Israeli Health Ministry-backed and peer-reviewed paper in the NEJM said the adjusted rate ratio of confirmed cases for Israelis over 60 at least 12 days after receiving a booster shot was lower by a factor of 11 compared with those who had had only two shots. The authors said they found the rate of severe illness among those over 60 who got the booster shot was lower by a factor of around 20 times, compared with those who hadn’t received the boosters.

Already, the country’s scientific community is again moving out in front of counterparts elsewhere by discussing the possibility of recommending further booster shots in the months and years to come.

“There will be a fourth, fifth, sixth or even seventh shot as long as Covid-19 continues to strike the world…we’ll continue to see variants rising up,” said Dr. Afek.

Write to Thomas Grove at thomas.grove@wsj.com