Exemptions for immunizations required in school are on the rise in the U.S., leading to concerns among medical experts that diseases like measles could soon make a comeback in many states.

In November, the Centers for Disease Control and Prevention reported that exemptions for immunizations required in school is the highest ever recorded in U.S. history – increasing to an average 3.0% in the 2022-2023 school year, with 10 states now reporting exemptions exceeding 5%. This leaves both vaccinated and unvaccinated children vulnerable to disease outbreaks including measles, experts say.

"There's 9 million people in this country who can't be vaccinated. They depend on those around them to protect them," Dr. Paul Offit, a virologist and vaccine advocate, told ABC News.

In his upcoming book, "Tell Me When It's Over: An Insider's Guide to Deciphering Covid Myths and Navigating Our Post-Pandemic World," Offit gives a historical account of the anti-vaccination movement in the U.S. and explains the rise of non-medical immunization exemptions over time and how these trends relate to the nationwide pushback against COVID-19 vaccine mandates.