The halt on third doses should be in place until at least the end of September, Director-General Tedros Adhanom Ghebreyesus said at a media briefing on Wednesday. That would help achieve the WHO’s goal to vaccinate at least 10% of the population in every country by that date, protecting health-care workers and vulnerable people.

“I understand the concern of all governments to protect their people from the Delta variant, but 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,” Tedros said.

He also called on vaccine producers to prioritize Covax, the program set up last year to equitably distribute vaccines to every corner of the planet. Many lower-income countries rely on Covax, but the initiative has delivered a fraction of the 1.8 billion doses it aims to ship by early 2022. High-income countries have received almost one dose for every person—although most vaccines require two doses for full immunization—compared with low-income countries, which have only been able to administer 1.5 doses for every 100 people, Tedros said.

After an early and effective vaccination drive beginning in December, Israel became the first country last week to widely roll out booster shots to halt a resurgent spread of the virus. A growing number of countries, such as Sweden and Greece, are studying this possibility.

“We don’t have a full set of evidence around whether this is needed or not,” said Kate O’Brien, who heads the WHO’s vaccination division. “This is a really big decision. It’s important we’re clear about what the data are, and if we’re not really grounded in that clarity, we’ll be in a place where we have forever uncertainty about what actually should be done.”

The main reason for the growing number of infections isn’t the failure of vaccines, but rather the easing of other health measures, she added.

“We need instead to focus on those people who are most vulnerable, most at risk of severe disease and death, to get their first and second doses, and then move on to advanced programs as the evidence gets stronger and as supply is assured,” O’Brien said.