Why Monkeypox Vaccine Shortage May Threaten the Immunocompromised

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. Now the shortage has led federal officials to consider a rarely used approach: a so-called dose-sparing strategy, which gives shots that each contain just one-fifth of a single dose.

For most recipients, one shot should be enough to ward off serious illness, and there is some evidence that even smaller doses can be effective. But preliminary research suggests that people with H.I.V. or other conditions that weaken the immune system may be less protected than people who don’t have such illnesses, according to some experts.