US measles cases have surged to 5,182 this year, the highest annual total since 1991, according to data released by the Centers for Disease Control and Prevention on Friday.

The figure more than doubles the 2,289 cases reported for all of 2025. The vast majority of infections, 96 percent, occurred among people who were unvaccinated or whose vaccination status was unknown.

Pennsylvania remains the epicenter of the outbreak, with state health officials reporting 1,487 cases and 287 hospitalizations as of Friday. The state also confirmed its sixth measles-associated death this week. South Carolina and Utah recorded the next highest case counts at 670 and 536, respectively.

Nationwide, hospitalizations linked to measles reached 562, or 11 percent of all reported cases. Adults aged 20 and older accounted for 2,132 cases, followed by 2,104 cases among those aged 5 to 19 and 908 cases in children under 5.

State health departments reported additional deaths this week, including an infant in Wisconsin and the first known measles-associated death in New York in more than 30 years. The CDC has recorded four such deaths nationally so far this year.

Measles was declared eliminated in the United States in 2000. The current surge has put that status at risk, with health officials noting sustained transmission that could lead to the loss of the elimination designation later this month.

Outbreaks have been reported in 43 locations across 47 jurisdictions. Most cases this year stem from ongoing transmission that began in 2025, though new outbreaks continue to emerge.

The highly contagious virus spreads rapidly in communities with lower vaccination coverage. Officials have linked the rise to vaccination rates that have not fully recovered from declines during the COVID-19 pandemic.