The CDC reports 451 hospitalisations nationwide as health officials expand vaccination efforts.

The United States Centers for Disease Control and Prevention (CDC) has recorded 4,410 measles cases so far in 2026, an increase of 330 in a week, and has dispatched an investigator to the state of Pennsylvania to help contain the country’s largest outbreak.

CDC figures also show 451 measles-related hospitalisations nationwide this year. Children and teenagers aged five to 19 account for 42 percent of cases, followed by adults aged 20 and older, who make up 41 percent. Seventeen percent of cases were among children under the age of five.

Pennsylvania reported 1,222 cases across 42 counties as of Wednesday, including 86 new infections since October 9, according to the state health department. The outbreak has led to 225 hospitalisations and five confirmed measles-associated deaths. State officials are actively investigating a sixth reported death.

The CDC is sending a so-called “disease detective” from its Epidemic Intelligence Service to work with state health officials on the Pennsylvania outbreak. The deployment follows Pennsylvania’s request for federal assistance, known as an Epi-Aid.

Pennsylvania has expanded vaccination efforts in affected communities. State health centre staff have administered more than 5,900 measles, mumps and rubella (MMR) vaccine doses at 202 pop-up clinics since the outbreak began in late April.

New York Governor Kathy Hochul declared a statewide disaster emergency on October 5 as measles cases rose in nearby Pennsylvania. The order is effective until November 4 and expands the number of healthcare professionals authorised to administer vaccines and allows nurses greater flexibility to order measles testing.

Measles vaccination coverage among US kindergarteners fell from 95.2 percent in the 2019–2020 school year to 92.4 percent in 2025–2026, according to data from the CDC. The agency estimates that about 280,000 kindergarteners are at risk because of gaps in vaccination coverage.