Montgomery County's three measles cases have held steady since April, but the statewide outbreak surrounding them has ballooned to 903 confirmed cases and four deaths, the Pennsylvania Department of Health reported Monday, Sept. 28.

The count stood at roughly 700 when the state confirmed its third and fourth deaths on Sept. 15. Thirteen new cases were added between Friday, Sept. 25, and Monday alone.

Cases have reached 39 counties. Lancaster County leads with 363, followed by Mifflin County at 106 and Chester County at 83, according to Patch citing state data. Neighboring Bucks and Delaware counties have reported zero cases.

Of the 903 patients, 176 have been hospitalized. The four deaths include two Lancaster County infants who were too young to be vaccinated, a 40-year-old unvaccinated woman in Jefferson County and an 18-year-old unvaccinated woman in Mifflin County, according to ABC News and county coroner reports. Mifflin County Coroner Andrea Alcalda said the 18-year-old's cause of death was acute disseminated encephalomyelitis, a rare neurological complication of measles.

The four deaths are the most recorded nationally in a single year since 1992, according to NationofChange citing CNN. They are also the first measles deaths in Pennsylvania in 35 years.

"We remain committed to working with our partners across Pennsylvania to stem this outbreak and prevent more deaths," Secretary of Health Dr. Debra Bogen said Sept. 15, when the latest deaths were confirmed, as reported by WHYY.

Less than 1% of the state's cases have occurred in fully vaccinated people. The majority of patients are between ages 25 and 49, and about 31% are children under 18, according to The Hill citing state health department data.

What Montgomery County residents should do

The Montgomery County Office of Public Health advises anyone who suspects measles to call a health provider, urgent care or emergency room before leaving home. Residents can also reach the county health office at 610-278-5117.

The two-dose measles, mumps and rubella (MMR) vaccine provides 97% lifetime protection. Anyone vaccinated before 1968 may need a new dose because older vaccines were less effective, the county health office said. Residents vaccinated in Pennsylvania can check their records through the state's PIERS Public Portal.

Since late April, health providers have administered more than 5,300 MMR vaccinations at 173 pop-up clinics in affected communities, according to WFMZ citing the state health department. In August, more than 49,000 MMR doses were given statewide. Over the first three weeks of September, another 38,000 doses were administered. Dozens of additional clinics are planned.

Measles spreads through the air when an infected person coughs or sneezes, and the virus can remain infectious for up to two hours after the person leaves an area, according to the state health department. Symptoms, including high fever, cough, runny nose and rash, can appear seven to 21 days after exposure. Exposed individuals who are not immune must quarantine for 21 days.

The state updates its measles dashboard at pa.gov/measles on Mondays, Wednesdays and Fridays. Residents can reach the state health hotline at 877-724-3258.