Following Measles Outbreaks, Officials Grow Wary of Renewed Threat

Measles, a highly contagious but preventable disease, is resurging in pockets of the United States, a warning of the dangers of the strengthening anti-vaccine movement.

The Centers for Disease Control and Prevention has recorded more cases this year than the 58 tallied in all of 2023, although the agency is not expected to release exact numbers until Friday. On Monday, the agency advised health care providers to ensure that unvaccinated patients, especially those traveling internationally, stay updated on their immunizations.

The number of cases is likely to keep rising because of a sharp spike in measles worldwide, along with spring travel to some regions with outbreaks, including Britain, said Dr. Manisha Patel, chief medical officer at the C.D.C.’s respiratory disease division.

Nearly all the cases in the United States so far are related to unvaccinated travelers. “We’re not going to see widespread measles cases going throughout the country,” Dr. Patel said. “But we do expect additional cases and outbreaks to happen.”

Measles is among the most contagious of diseases; each infected person can spread the virus to as many as 18 others. The virus is airborne and can stay aloft up to two hours after an infected person has left the room, spreading rapidly through homes, schools and child care facilities.

In Chicago, one case of measles at a migrant shelter has grown to 13, prompting the C.D.C. to send a team to help contain the outbreak. (Two additional cases in the city appear to be unrelated.)