The shortfall of monkeypox vaccine doses in the United States, expected to last for months, is raising urgent questions about how well and for how long a single shot may protect against the virus.

The vaccine, called Jynneos, is approved as a two-dose regimen, but most people at risk of infection have been receiving one dose — if they can find it.

So far, men who have sex with men account for virtually all monkeypox cases in the United States, and more than 40 percent are men living with H.I.V., according to an analysis by the Centers for Disease Control and Prevention.

Even two weeks after the shot, when the antibody response is underway, immunocompromised people still may need to “use all other precautions to prevent being exposed, per public health guidance,” she said.

Federal officials have ordered nearly seven million doses of Jynneos, but the shots will not arrive for months.

Faced with shortages, some cities, including Washington and New York, are restricting second doses to stretch their supplies.

have disagreed with that strategy, noting that Jynneos is approved as a vaccine to be given in two doses spaced 28 days apart.

But as federal health officials declared a public health emergency on Thursday, Dr?

The Jynneos vaccine, which protects against smallpox and monkeypox, can also help reduce symptoms, even if taken after exposure.

Jynneos vaccine is most commonly used to prevent monkeypox infections, and consists of two doses given four weeks apart.

It  has mostly been offered to health care workers and people who have had a confirmed or suspected monkeypox exposure due to limited supplies, though new doses should become available in the coming months.

That data derives not from trials of Jynneos, but from a small 1988 study that looked at the incidence of monkeypox among people who had been inoculated for smallpox earlier in their lives.

No large clinical trial of Jynneos as a monkeypox vaccine was conducted in humans before its approval.

relied on measures of antibody responses in small groups of people after immunization with Jynneos compared with those produced by ACAM2000, an earlier vaccine for smallpox.

Information about how Jynneos performs in people with H.I.V., particularly in those with severe immune problems, was already scant.

Yonts said the data from that trial was not conclusive, reduced antibody response is often seen among immunocompromised people given other vaccines.

“Individuals with severe or moderate immune suppression are recommended for additional doses of common vaccines,” said Keri Althoff, an epidemiologist at the Johns Hopkins Bloomberg School of Public Health, who led the Covid vaccine study.

and the New York City Department of Health say Jynneos is safe for people with H.I.V., but the agencies have not addressed its effectiveness in that population.

By contrast, health officials in Britain say that for people who “are H.I.V.

The vaccine’s package insert also notes that immunocompromised people “may have a diminished immune response.”

Chloe Orkin, an infectious disease physician at Queen Mary University of London, referring to immunocompromised people