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Measles was officially declared eradicated in the U.S. more than 20 years ago, but new outbreaks of the disease are popping up — and experts say declining vaccination rates are jeopardizing herd immunity and increasing the risk.

In Philadelphia, nine cases were reported after a cluster started in a hospital and spread to other medical facilities and a day care center. Measles is a highly contagious and potentially deadly virus that causes a tell-tale rash.

According to data from the Pennsylvania Department of Health, during the 2021-2022 school year, 94.3% of kindergarteners in Philadelphia County were fully vaccinated with the MMR vaccine, which protects against measles, mumps and rubella. Last school year, that dropped to 92.8% — below the 95% needed for herd immunity.

"That's really a wake-up call, because the real number in many communities is probably far below 93%," Dr. Peter Hotez, co-director of the Texas Children's Hospital Center for vaccine development and professor of pediatrics at Baylor College of Medicine, told CBS News.

He says outbreaks like this one are more likely when vaccination rates drop.