APOORVA MANDAVILLI

A factory making Jynneos, the safest vaccine, has been closed since August, threatening global supplies as the virus spreads.

States will be given vaccine doses from the federal stockpile, but supplies of the safest type are limited.

Even as cases rise, genetic analysis suggests that the virus has been silently circulating in people since 2018.

The vaccines seem safe for children and are likely to protect against severe illness. But data on efficacy are thin, and most children have already...

Some scientists believe that a clearer picture of Covid vaccine efficacy could have emerged sooner if investigators had tracked certain immune cells, not just antibodies.

The virus “is not known to linger in the air,” agency officials said. But the research is far from definitive.

An abrupt change in C.D.C. guidance underscores a little-known phenomenon: On occasion, monkeypox can be transmitted through aerosols, similar to the coronavirus.

As the number of cases outside Africa approaches 800, governments are scrambling for a limited pool of vaccines and treatments with unclear effectiveness.

Older people who received smallpox vaccinations may yet have some immunity, researchers say. Healthy children and adults generally do not become severely ill.

The United States has stockpiled millions of doses of two smallpox vaccines, also effective against monkeypox. But the outbreaks so far are clustered in other...