A cluster of measles cases originating at Children’s Hospital of Philadelphia (CHOP) in December has expanded to encompass at least three additional unvaccinated residents of the city, the Philadelphia Department of Public Health announced on Thursday.

Currently, four confirmed cases of measles have been identified, with two further potential cases under investigation. These latter cases involve individuals exposed to a confirmed measles case while the infected person was in violation of quarantine orders at a local daycare center.

Measles, a highly contagious viral illness transmitted through airborne respiratory droplets or contaminated surfaces, as outlined by the Centers for Disease Control and Prevention (CDC), poses a significant health risk, particularly for individuals under five years old, pregnant women, and those with compromised immune systems.

The locations with cases include the Jefferson Health building, Multicultural Education Station Day Care, CHOP emergency department, St. Christopher’s Hospital for Children, and Nazareth Hospital emergency department.

Unvaccinated individuals exposed to measles are advised to self-isolate for 21 days. The MMR vaccine effectively protects against measles, mumps, and rubella. Those who have received the vaccine are not considered at risk of exposure.

The Department of Public Health urges any unvaccinated individuals present at any of the identified exposure locations, either confirmed or suspected, and who lack immunity to measles through other means, to contact their healthcare provider promptly. This includes children under 15 months who have not yet received the recommended MMR vaccination.