Pertussis outbreaks are at the highest level in a decade and doctors say many cases are going undetected — allowing people to spread the bacterial infection unknowingly for weeks.

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.

“With the increase in vaccine hesitancy that has been going on since the Covid-19 pandemic, we’re seeing outbreaks occurring in kids who are not vaccinated,” said Dr. Tina Tan, president-elect of the Infectious Diseases Society of America.

Babies are given the DTaP vaccine, which helps protect against three diseases: pertussis, diphtheria and tetanus. The vaccine works well against diphtheria and tetanus, but is less effective over time for pertussis.