Whooping cough cases are climbing at the fastest pace in years as students across America return to school, U.S. health officials reported Thursday.

U.S. health officials have warned for months about a resurgence in breakthrough infections in older children and adults, even though unvaccinated young children and newborns of unvaccinated moms remain the most vulnerable to both infection and severe disease, CBS News reported.

The U.S. is experiencing more than four times as many whooping cough cases compared with last year — a spike that some experts attribute to post-pandemic vaccine fatigue.

The resurgence of whooping cough cases in the US this year now accelerated to the fastest pace on record in nearly a decade, according to the Centers for Disease Control and Prevention. Also known as pertussis – this bacterial infection is now again climbing around the country during the back-to-school season.