The World Health Organization on Wednesday called for a halt on booster shots of coronavirus vaccines through at least September, as poorer countries struggle to access doses, even for high-risk populations such as health-care workers and the elderly.

“We cannot and we should not accept countries that have already used most of the global supply of vaccines using even more of it while the world’s most vulnerable people remain unprotected,” WHO Director General Tedros Adhanom Ghebreyesus said at a news conference.

Tedros said the focus for now should be meeting the U.N. health agency’s goal of 10 percent vaccination coverage in every country by the end of September. So far, more than 80 percent of vaccine doses globally have gone to high- and upper-middle-income countries that account for less than half of the world’s population.

The remarks come as the United States and other wealthy nations weigh whether and when booster shots are necessary and consider how to balance domestic calls for additional doses against growing evidence of an alarming vaccine gap around the world.

White House press secretary Jen Psaki on Wednesday said the WHO was presenting a “false choice.”

The United States announced Tuesday “an important milestone” of more than 110 million vaccine doses donated to the world, she said, more than all other countries have shared, combined.

“More needs to happen, but we believe we can do both,” she said.

In recent months, as a small number of relatively rich countries have pressed ahead with coronavirus vaccination campaigns, the WHO, public health experts and advocates have urged rich countries to do more to share doses and increase global supply.

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

Now, the spread of the highly-transmissible delta variant has countries with vaccine access considering third doses before much of the world has had access to one.

The WHO’s position on boosters is still taking shape. WHO officials said Wednesday that they do not necessarily oppose giving additional doses to certain populations who are not protected by standard doses.

Bruce Aylward, senior adviser to the director general, acknowledged that the moratorium may also need to be extended. “Is September long enough? Not on the current trajectory,” he said.

Officials in the United States say booster shots are not needed yet. But there is growing concern among some health officials about the urgent need to give additional doses to people with fragile immune systems amid growing concerns about waning immunity in vulnerable populations and surging infections from delta.

Some patients are trying to get additional doses, including those who are immunocompromised and are doing so at the recommendation of their doctors, clinicians have said.

Many patients have “taken matters into their own hands and many are proceeding with additional doses of vaccine as they see fit,” Camille Kotton, a transplant medicine physician at Massachusetts General Hospital in Boston and a member of the vaccine advisory board to the Centers for Disease Control and Prevention, said at the panel’s meeting last month.

In Northern California, the Contra Costa County health department notified its providers July 23 to allow people who wanted a booster dose of any coronavirus vaccine to get one if one was available, only to reverse itself this week after realizing it was in violation of FDA policy, a spokesman said.

The FDA’s existing authorization permits only a two-dose regimen of the Pfizer-BioNTech and Moderna vaccines, or a single dose of the Johnson & Johnson product. But the FDA is expected to give full approval to the Pfizer vaccine by the end of the summer, according to a federal official familiar with the situation who spoke on the condition of anonymity because the process is ongoing.

Once the Pfizer vaccine is licensed, clinicians could recommend additional doses to immunocompromised patients and other vulnerable people who are more likely to become seriously ill from covid-19 and might more frequently spread the virus to others.

Emerging data suggests that an additional dose in immunocompromised people may enhance disease-fighting antibody responses and increase the proportion of those who respond to the vaccines, CDC officials have said.

Debate over the need for booster shots has intensified since Pfizer and BioNTech announced last month that they would seek regulatory approval for a third shot for all eligible adults, not just the immunocompromised, amid rising global concern about the delta variant.

The companies’ announcement that they would seek an emergency-use authorization for a booster prompted U.S. health officials to declare that Americans who have been fully vaccinated do not need a third shot at this time.

But health officials make a distinction between boosters for people who already have an immune response after getting vaccinated versus additional doses for the immunocompromised who have failed to respond fully or at all to the shots.

Other countries are grappling with questions over boosters. A booster campaign is already underway in Israel, where the effort focuses on adults over 60, and Russia. Clinics in Moscow started offering boosters in July, amid a surge in cases and concern about delta.

Parts of Europe appear to be moving in that direction. The WHO’s announcement came two days after Germany said it also would start offering boosters to the elderly, the immunocompromised and those who got the AstraZeneca or Johnson & Johnson shots, because they may be less effective than other vaccines.

France also plans to make certain groups of people — residents of nursing homes, those over age 75 and people with severe health conditions — eligible for boosters in September.

Hungary this month will begin offering citizens a third dose of a coronavirus vaccine, Prime Minister Viktor Orban has said. Both Spain and Italy have said residents will be very likely to need a booster but have yet to announce firm plans.

In Britain, meanwhile, health officials have said they are preparing to offer booster shots in September but are awaiting guidance from an expert advisory panel before firming up plans.

With global supply scarce, the booster debate will deepen questions about how the world should address the problem.

Since last summer, a small number of relatively wealthy nations have cut deals with directly with vaccine manufacturers, snapping up a disproportionate share of near-term supply and undermining a WHO-backed effort, known as Covax, to equitably distribute vaccine.

Covax aimed to deliver up to 2 billion doses this year, with an eye to vaccinating 20 percent of the populations of participating countries in need. To date it has delivered just under 178 million doses to 138 countries.

As it became clear that the United States had more than enough doses for its population, the Biden administration stepped up its vaccine diplomacy, promising to play a leadership role.

At a June meeting of the Group of Seven in Britain, the White House announced it would buy 500 million doses of Pfizer’s coronavirus vaccine to donate to the world, a step President Biden said would help “help supercharge the global fight against the pandemic.”

On Tuesday, Biden said the United States has sent more than 110 million doses of coronavirus vaccines to 65 countries, putting the country ahead of all others on vaccine donations.

Other wealthy nations have pledged to share surplus doses, but supply remains limited — and diverting supplies to boosters could make that worse, the WHO warns.

Asked if nations already providing extra doses are “failing their duties,” WHO officials focused Wednesday on those still pondering the issue, saying they hoped leaders will hold off.

Tedros urged “concrete” commitments to global vaccination goals and ventured that the leaders of G-20 countries will determine the course of the pandemic.

“We need everyone’s cooperation,” he said.

Reis Thebault contributed to this report.