Pennsylvania state health officials have confirmed 1,004 cases of measles this year, marking the state's largest outbreak in over three decades. Five residents have died after contracting the disease, and 198 have been hospitalised.
Lancaster county has been the most affected area, reporting 391 cases. The highest number of infections has been observed in the 24-49 age range, with case numbers increasing over the past three months.
The outbreak, which began in April west of Philadelphia, has been attributed by health officials to low vaccination rates. Governor Josh Shapiro has expanded access to the measles, mumps, and rubella (MMR) vaccine in affected communities.
Neighbouring New York state declared a state of emergency on Monday due to 108 cases as of 3 October. Ohio has also reported over 200 cases this year.
The US is experiencing its worst year for measles since 1991, with the CDC reporting 3,887 confirmed cases nationwide as of 1 October. Outbreaks have also been reported in states including Texas, Utah, and South Carolina.