The United States Centers for Disease Control and Prevention (CDC) has recorded 4,410 measles cases so far in 2026, with 451 related hospitalisations reported nationwide.

The weekly case count rose by 330, prompting the CDC to dispatch an investigator to Pennsylvania to help contain what has become the country's largest outbreak.

Children and teenagers aged five to 19 account for 42 percent of cases. Adults aged 20 and older make up 41 percent, while children under five represent the remaining 17 percent.

Pennsylvania has reported 1,222 cases across 42 counties as of Wednesday, including 86 new infections since October 9. The state outbreak has resulted in 225 hospitalisations and five confirmed measles-associated deaths. Authorities are actively investigating a sixth reported death.

The CDC is deploying a so-called "disease detective" from its Epidemic Intelligence Service to work alongside Pennsylvania state health officials. The deployment follows Pennsylvania's formal request for federal assistance, known as an Epi-Aid.

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 continued to rise in neighbouring Pennsylvania. The order, effective until November 4, 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 CDC data. The agency estimates that approximately 280,000 kindergarteners are currently at risk due to gaps in vaccination coverage.