On September 3, the New York City Department of Health and Mental Hygiene officially declared the end to the 2018–19 measles outbreak, which accounted for more than 650 cases in the 5 boroughs—the most in New York City in more than 3 decades. However, political leaders and health professionals warn that another outbreak could occur if parents do not follow proper protocols for vaccinating their children.

In an opinion piece in The New York Times, Jennifer L. Lighter, MD, a pediatric infectious disease physician and epidemiologist at Hassenfeld Children’s Hospital at NYU Langone and assistant professor in the Department of Pediatrics, points to a principal reason for the recent measles outbreak—and it might surprise you. She says that the problem was not solely with science denial or the anti-vaccine movement, but rather more with those often referred to as the “vaccine hesitant.”

“Rather than following recommended scheduling for measles vaccination, parents choose to follow their own schedule, and some pediatricians supported these efforts,” she says. “As a result, many children were under-vaccinated, and that left them vulnerable to the disease.”

Read more from Dr. Lighter in The New York Times.