More than 4,400 measles cases have been reported in the U.S., according to an update on Friday from the Centers for Disease Control and Prevention (CDC).
A total of 4,410 cases have been confirmed, with most reported by 45 states and Washington, D.C. Eighteen of this year's total have been among international visitors to the U.S., according to the CDC.
At least 1,200 of those cases have been reported in Pennsylvania alone by local health officials, as have six measles-related deaths.
It comes as the CDC is planning to send a "disease detective" to Pennsylvania in response to the outbreak.
"Pennsylvania has formally requested CDC assistance through the Epi-Aid program. In response, CDC will support the state's identified needs and strengthen its response efforts on the ground," a CDC spokesperson told ABC News in a statement.
A CDC Epi-Aid is a rapid, short-term field investigation from the federal health agency of an "urgent public health problem." The Epi-Aid provides resources such as, but not limited to, increasing the workforce available for rapid response and building epidemiologic capacity, according to the CDC.
In this Oct. 24, 2025, file photo, a vial of the Measles, Mumps, and Rubella (MMR) vaccination for children is displayed during an immunization event in Los Angeles.
Patrick T. Fallon/AFP via Getty Images, FILE
"CDC will continue working closely with Pennsylvania health officials to provide the expertise and support needed to respond to the outbreak and protect public health," the statement continued.
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Fewer than 1% of the state's cases are among those who are fully vaccinated with the MMR vaccine, according to the Pennsylvania Department of Health.
This tracks with national trends. The CDC shared in it its update that about 96% of cases are among people who are unvaccinated or whose vaccination status is unknown, according to agency data.
Meanwhile, 2% of cases are among those who have received just one dose of the measles, mumps, rubella (MMR) vaccine and 2% of cases are among those who received the recommended two doses, according to the CDC.
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. One dose is 93% effective, and two doses are 97% effective against measles, the CDC says.
However, CDC data shows vaccination rates have been lagging in recent years. During the 2025-26 school year, 92.4% of kindergartners received the MMR vaccine, according to data. This is lower than the 95.2% seen in the 2019-20 school year, prior to the COVID-19 pandemic.
The CDC said that Health Secretary Robert F. Kennedy Jr. and CDC Director Dr. Erica Schwartz "continue to recommend the MMR vaccine as the most effective way to prevent measles and protect against infection."