Federal and Pennsylvania health officials confirmed a fifth measles-related death on Thursday, marking the deadliest measles episode in the United States in more than three decades. The Pennsylvania Department of Health identified the latest fatality as an unvaccinated resident.

The announcement followed a fourth death reported earlier this week. State officials linked both cases to low vaccination coverage in several communities, with confirmed cases continuing to rise across multiple counties and additional infections identified in neighbouring states.

CDC officials reported that the majority of cases involved people who were unvaccinated or whose vaccination status was unknown. The agency urged parents to ensure children received two doses of the measles, mumps and rubella (MMR) vaccine, which it said was 97% effective against infection.

The deaths intensified pressure on state and federal authorities to expand outreach in undervaccinated areas. Pediatric infectious disease specialists at the Children's Hospital of Philadelphia warned that declining immunisation rates had left pockets of the population vulnerable to rapid spread.

"Every one of these deaths was preventable," said a Pennsylvania Department of Health spokesperson. "We are calling on every family to check their vaccination records and act now before this outbreak grows further."