Pennsylvania's measles outbreak has reached 1,222 confirmed cases and at least five deaths, with neighboring Chester County now one of the largest clusters in the state.

Chester County reported 90 cases as of Wednesday, Oct. 14. That is the third-highest total in Pennsylvania, behind Lancaster County's 443 and Mifflin County's 183, the Philadelphia Inquirer reported. Chester County is the only county in the Philadelphia region where the virus is known to be actively spreading, according to Medical Daily.

Chester County shares a border with Delaware County, which includes Radnor and Haverford townships. Delaware County had reported zero measles cases as of Oct. 5, according to NBC10.

The state is also investigating a sixth death. The Franklin County Coroner's Office reported that a 39-year-old woman with several underlying conditions, including a past measles diagnosis, had died, WHYY reported. The Pennsylvania Department of Health confirmed the five earlier deaths in Jefferson, Mifflin and Lancaster counties, with Lancaster accounting for three, according to 6abc.

About one in five people who contracted measles required hospitalization, state data shows. More than 99% of cases involved unvaccinated individuals, according to 6abc.

The outbreak prompted the U.S. Centers for Disease Control and Prevention (CDC) to send a specialist from its Epidemic Intelligence Service to Pennsylvania the week of Oct. 14, the Inquirer reported. Federal officials also committed nearly $4 million to support measles response operations through most of 2027, according to a U.S. Department of Health and Human Services official.

Infections have spread to 42 of Pennsylvania's 67 counties, WHYY reported. The largest clusters of community-wide viral transmission are concentrated in and around Lancaster, Chester, York, Mifflin, Snyder, Centre, Clinton, Clearfield and Jefferson counties.

The Chester County Health Department warned residents of possible measles exposure at two CVS stores and a Giant supermarket along Baltimore Pike in Kennett Square and West Grove on Oct. 4. Oxford Area High School also closed Oct. 9 after a confirmed case, with unvaccinated students and staff facing a 21-day quarantine.

Jeanne Franklin, public health director of the Chester County Health Department, has led an outreach campaign that includes home visits to vaccinate residents. In an Oct. 13 interview with ABC News, Franklin described the county's approach: "We average about 10 people per call but have vaccinated as many as 67 in one location."

Statewide, healthcare providers administered more than 56,000 measles, mumps and rubella (MMR) vaccines in September, more than double a typical month, the Inquirer reported. Providers gave out more than 23,000 additional doses in the first two weeks of October.

The two-dose MMR vaccine is about 97% effective at preventing infection, according to the CDC. Infants as young as six months can receive an early dose during outbreaks, though it does not count toward the regular two-dose series required for school, according to the Pennsylvania Department of Health.

Residents who suspect a measles infection should call their doctor or local health department before visiting a healthcare facility.