Childhood vaccine exemptions are at an all-time high. Doctors worry diseases like measles could return

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.

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.

ABC News spoke with Offit and other doctors who work in infectious diseases and immunization research to explore what's driving these trends and understand what the future may hold if immunization exemptions keep rising.