The emergency use authorization also allows for the vaccination of children who are at high risk of monkeypox, according to a news release issued by the agency. Prior to the announcement, only those ages 18 and older could be vaccinated without patient-specific approval from the FDA.

“In recent weeks the monkeypox virus has continued to spread at a rate that has made it clear our current vaccine supply will not meet the current demand,” FDA Commissioner Dr. Robert Califf said in the release. “By increasing the number of available doses, more individuals who want to be vaccinated against monkeypox will now have the opportunity to do so.”

The authorization was issued Tuesday because the now globally circulating pathogen “has a significant potential to affect national security or the health and security of United States citizens living abroad,” according to the release.

As of Monday there had been nearly 9,000 cases of monkeypox identified in the U.S. since January, with cases in every state except Wyoming. Globally there had been more than 30,000 cases identified in that period, with only 345 being identified in countries that have historically reported the disease.

Dr. Georges Benjamin, executive director of the American Public Health Association, told Fortune on Tuesday that the move may allow the U.S. to gain ground in an outbreak it’s behind on, partially due to an inadequate vaccine supply.

“The fact is we want to make sure we have enough vaccine,” he said. “We’re chasing this thing. To the extent they’re doing it something that allows them to fairly dramatically increase the amount of vaccine, this will help us play catch-up.”

The FDA on Thursday had announced its consideration of the supply-stretching plan, referred to as “dose sparing,” which would split each dose into five and involve injection intradermally, or between layers of the skin, versus subcutaneously, or under the skin.

That announcement came just moments after Department of Health and Human Services Secretary Xavier Becerra declared a public health emergency over the globally circulating virus, once endemic only to Africa.

Califf on Thursday said the approach provides improved immune response and does not jeopardize safety or effectiveness.

Though Jynneos—one of two smallpox vaccines approved for fellow poxvirus monkeypox, and the safest by far—is authorized by the FDA for treatment of monkeypox in adults, an emergency use authorization was needed due to changes in dosing and administration, as well as to allow children to receive the vaccine.

‘Safe and efficient mass vaccination’

While “dose sparing” might sound like a desperate solution to inadequate vaccine supply, it’s “not being done randomly.” There is data behind the concept, Dr. Lindsey Baden, an infectious disease specialist at Brigham and Women’s Hospital in Massachusetts and a Harvard Medical School professor, told Fortune on Friday.

A prior study of dose sparing involving smallpox vaccine showed that a fifth of a dose—and perhaps even a tenth—produced an immune response comparable to a whole dose, Baden said.

And there’s a “whole body of science” on the superiority of immune response after intradermal injections, according to Benjamin.

Administering a small dose of smallpox vaccine intradermally allows for “safe and efficient mass vaccination, a prerequisite when vaccination of a large population at a short time is needed,” the authors of a 2018 study published in the journal Vaccines reported.

The approach is “definitely worth exploring,” Dr. Alexandra Brugler Yonts, an infectious disease specialist at Children’s National Hospital in Washington, D.C., who assisted in the FDA’s review of Jynneos, told Fortune on Friday.

But “there isn’t much available data specifically for Jynneos,” she cautioned.

“Any studies on this route of delivery were not evaluated as a part of the initial licensure of Jynneos in 2019, which is why intradermal injection is not included on the current package insert,” she said. “But if the FDA has that data now to review and find it convincing, it would be a reasonable temporizing measure.”

She said she was concerned that smaller doses may not be adequate for those with HIV or those who are immunocompromised.

“Some protection is better than none, but the people being vaccinated should be warned and advised to continue other protective measures to minimize physical or intense close contact with potentially infected individuals,” she said.

“Just like with COVID, receiving the vaccination—especially only a single dose or smaller doses—is not a free pass to blindly take part in high risk behavior in the context of an active outbreak.”

Baden said he’s concerned with the safety and efficacy of every treatment—a dose-sparing approach to Jynneos included—and thinks more data are always better.

“We can always want more science,” he said. “But more doesn’t mean you haven’t already demonstrated the point.”

A blanket authorization for childhood vaccination

Illinois health officials Friday said the FDA had authorized the use of Jynneos for potentially exposed children at a Champaign County daycare center, where a worker tested positive for monkeypox.

An FDA spokesperson told Fortune via email on Friday that the vaccine was being approved for such children via a “single patient expanded access investigational new drug application” submitted for each. Applications are processed “as quickly as possible” and approved “when no comparable or satisfactory alternative options are available and requested by the treating licensed physicians, who determine whether the benefits outweigh the probably [sic] risk.”

The spokesperson at the time would not comment further on the possibility of general approval for the use of Jynneos in children.

Because Jynneos is licensed, not just authorized under an emergency use authorization as initial COVID vaccines were, it can be used “off label”—for instance, given to children who have been exposed, Brugler Yonts said Friday.

Researchers “will certainly want to collect safety and, if possible, immunogenicity data on these kids and follow them closely, since there is no preexisting data for use in the pediatric population,” she said. “But Jynneos is safe, and if this can prevent a larger outbreak in the pediatric population—and of course the adults that care for them and live with them—that is very important.”

The decision to provide the vaccine to children is “worth the potential risk,” she added.

The vaccine will be given to children subcutaneously, the traditional route, the FDA’s Tuesday release said, though it didn’t state the size of the dose that would be given.