TEL AVIV - Israel's Ministry of Health on Monday began offering a third dose of the Pfizer coronavirus vaccine to severely immunocompromised adults in what health experts say could be the first phase of an experiment to provide coronavirus booster shots for older people and the most vulnerable. 

The recommendation, published Sunday by the ministry, said the goal of the new program was to raise antibody levels among immunocompromised citizens, including cancer patients, recipients of liver transplants, and others who have recently exhibited weakened vaccine protection, according to data. It said that it had still not made a decision on administering third shots for the general adult population.

Globally, the push to introduce booster shots has prompted pushback from the World Health Organization (WHO) and rights groups, who say the focus should remain on getting first doses to the world's most vulnerable.

The decision to offer some people third doses comes as Israel, which was among the fastest countries to vaccinate in the winter and then among the first to begin reopening in the spring, is experiencing a surge in new cases, spurred by the prevalence of the highly transmissible delta variant, first identified in India. Over the past month, infection rates in Israel have spiked from single digits to more than 400 a day.

And on the same day, Pfizer officials met with top U.S. federal health officials to make their case for administering some Americans - particularly the elderly and the immunocompromised - a third dose six to 12 months after receiving the companies' two-shot regimen.

That meeting came after the Department of Health and Human Services publicly rebuked Pfizer when it and the German company BioNTech announced last week they planned to seek an emergency use authorization for its booster shot. HHS said that fully vaccinated Americans do not need a booster for now.

During the meeting on Monday, Pfizer officials provided U.S. health officials a similar briefing to one they gave the Europeans this month. The White House meeting was largely focused on the science and data Pfizer presented. The company cited data from Israel showing a rise in infections among the vaccinated population, as well as interim data from the company's trial of its booster shot showing a third dose stimulates a much stronger antibody response that is five to 10 times the level seen after the second dose of its vaccine.

There were no decisions made about how to proceed, and officials acknowledged there is still a significant amount of data that has yet to be evaluated, according to a person with knowledge of the meeting.

It remained unclear on Monday where U.S. officials stood on the need for a third shot for vulnerable Americans. While several senior officials believe it will be appropriate to recommend boosters for the elderly and immunocompromised, Pfizer still must receive emergency authorization from the Food and Drug Administration for its third dose and a CDC advisory panel must decide whether and to whom to recommend boosters. That process could take several weeks or months.

Pfizer officials and some U.S. health officials are worried that if the U.S. government takes too long to make a decision on whether to begin administering another dose of the vaccine, the delta variant will cause another surge of the virus this fall among the unvaccinated and could infect the vulnerable who are vaccinated.

An HHS spokesperson who spoke on the condition of anonymity to share information on an internal meeting said health officials are briefed routinely by manufacturers and others on the latest data on coronavirus vaccines. The spokesperson also reiterated that, "at this time, fully vaccinated Americans do not need a booster shot," but added that the administration is prepared for booster doses "if and when the science demonstrates that they are needed."

But the discussion of booster shots has also raised concerns about the impact it could have on vaccine hesitancy, as well as questions about the ethics of providing fully vaccinated residents of wealthy countries a third shot when the majority of the world has yet to receive a single dose.

The director general of the World Health Organization, Tedros Adhanom Ghebreyesus, in a statement Monday admonished vaccine manufacturers for seeking to push booster doses to wealthy countries when many places still do not have access to doses.

"The global gap in vaccine supply is hugely uneven and inequitable," he said at a news briefing.

In recent months, as a small number of relatively wealthy countries have pressed ahead with vaccination campaigns, the U.N. health agency, public health experts and advocates have warned of a widening global vaccine gap and urged governments to do more to share doses and increase supply.

They argue that unequal distribution of doses is not only unethical but also could extend the pandemic by prolonging shutdowns and giving the virus room to spread and mutate in unvaccinated populations.

"We will look back in anger, and we will look back in shame if countries use precious doses on booster shots, at a time when vulnerable people are still dying without vaccines elsewhere," Mike Ryan, head of the WHO's emergency program, said Monday.

But Pfizer officials have pointed to worrisome data in Israel as a case study. Just weeks after lifting most COVID restrictions, the government has reinstated the mask mandate for indoor spaces and public transportation. It is expected to introduce stricter quarantines for travelers returning from abroad and rapid testing stations for students, and to revive the recently retired "green pass" system granting vaccinated people broader access to such public events as concerts and movie showings.

Pfizer and its partner BioNTech last year sold Israel millions of doses, which were delivered on cargo planes that were greeted with fanfare by then-Israeli Prime Minister Benjamin Netanyahu at Ben Gurion Airport. Pfizer views Israel - with its small size, heterogenous population and meticulously digitized national health care system, which serves as the basis for a data-sharing agreement signed by the Israeli government and the pharmaceutical giant - as a test case for vaccine rollouts in the rest of the world.

Recent studies show that the Pfizer vaccine remains effective against the delta variant in preventing hospitalizations and serious illness, though it also has shown declining effectiveness at preventing milder cases. The company expects to publish data from the current study on booster shots provided to at-risk adults in Israel. It said Thursday that it will ask U.S. and European regulators within weeks to authorize booster shots.

Israeli Prime Minister Naftali Bennett announced that he has coordinated a fast-tracked delivery of the next batch of Pfizer doses to arrive Aug. 1, to allow the country to replenish its dwindling supplies and continue its campaign to inoculate 12-to-15-year-olds.