The city of Philadelphia is blaming the recent measles outbreak on people declining to get vaccinated and failing to follow quarantine recommendations, after its Department of Public Health confirmed at least four cases.

The city’s Department of Public Health has confirmed at least four cases and is investigating two other possible infections.

“Unfortunately, we are seeing cases of measles that have spread to vulnerable individuals including young children due to people declining vaccination and also failing to adhere to quarantine recommendations,” Cheryl Bettigole, Philadelphia’s health commissioner, wrote in a Thursday press release. “Philadelphia is a city where we believe in a duty to take care of each other.”