(NEW YORK) -- At least 13 new measles cases have been confirmed in Pennsylvania since Friday, bringing the total so far this year to 903, according to Monday's update from the state health department.

Cases have been recorded in 39 of Pennsylvania's 67 counties. At least 176 people have been hospitalized, and four measles-associated deaths have been reported, the Pennsylvania Department of Health (DOH) said.

Two of the deaths were among unvaccinated adults: an 18-year-old and a 40-year-old. The other two deaths were among infants who were too young to have received the measles, mumps, rubella (MMR) vaccine.

Fewer than 1% of the state's cases -- four in total -- are among those who are fully vaccinated with the MMR vaccine, according to DOH.

The majority of cases are among those between ages 25 and 49, followed by those between ages 18 and 24, according to state health department data.

Lancaster County in southeastern Pennsylvania has recorded the highest number of cases at 363 followed by Mifflin County, in the central part of the state, with 105.

At least 11 counties show evidence of community transmission, which is when infectious illness spreads through a local population without any known travel history and the specific source of infection cannot be determined.

DOH data shows staff have administered more than 8,300 doses of the MMR vaccine so far this year. Uptake was highest in August with about 3,500 vaccine doses administered, followed by September with about 2,500 doses administered.

Meanwhile, measles cases in the U.S. have surpassed 3,600 so far in 2026, with cases confirmed in 45 states and Washington, D.C., according to the latest data from the Centers for Disease Control and Prevention.

The CDC has reported one death from measles so far this year but did not say where the death occurred. A note on the agency's website said the number "is subject to change."

The CDC currently recommends that people receive two doses of the MMR vaccine, the first at ages 12 to 15 months and the second between 4 and 6 years old. Infants who are between 6 and 11 months old and traveling outside of the country may require an additional vaccination. One dose is 93% effective in preventing measles infection and two doses are 97% effective, the CDC says.

However, CDC data shows vaccination rates have been lagging in recent years. During the 2025 to 2026 school year, 92.4% of kindergartners received the MMR vaccine, according to data. This is lower than the 95.2% seen in the 2019 to 2020 school year, prior to the COVID-19 pandemic.

"If you've been exposed, it is important to talk to a healthcare professional, but don't go into an urgent care or an ER because you may be inevitably spreading it to other vulnerable people," ABC News medical contributor Dr. Alok Patel told "ABC News Live" on Friday.

"People who have been exposed may be instructed to quarantine or keep really close eye on their symptoms for up to 21 days because that's how long it can really take for measles to show itself and you could potentially spread and infect others without even knowing it," Patel added.

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