HOLD - Deadly illness spreading twice as fast in 2025, surpasses 9,000 cases

(NEXSTAR) – It's not just measles making a fierce comeback in 2025. Another disease that's especially dangerous to young children, known as pertussis or whooping cough, is spreading nearly twice as fast as it was last year.

As of April 26, the latest data available from the Centers for Disease Control and Prevention, 9,047 cases have been identified in U.S. residents in 2025. That's nearly twice as many as there were at this time last year, when about 4,700 cases had been identified.

Whooping cough tends to peak in the spring and fall. It's usually spread through respiratory droplets in the air when people with pertussis cough, sneeze or breathe close to others. The symptoms are similar to a cold, but the cough becomes increasingly severe with a distinctive sound — a “whoop” as the person tries to take in air.

The illness is most dangerous for infants too young to get the vaccine.

Experts say the outbreaks of vaccine-preventable illnesses, like measles and whooping cough, could be indicative of changing attitudes toward vaccines. U.S. kindergarten vaccination rates fell last year, and the number of children with vaccine exemptions hit an all-time high.

Where is whooping cough spreading fastest?

West Coast states represent the greatest share of cases thus far in 2025. Washington has reported more than 1,000 cases alone. Oregon has had 663 cases and California has had 590 cases of whooping cough, according to the CDC.

Another cluster of states reporting high numbers is Michigan (537), Ohio (494), Wisconsin (346) and Illinois (319).

Florida also has at least 468 cases of whooping cough in 2025.

Why is whooping cough spreading again?

One leading theory for the recent increase in pertussis cases is the recent decrease in vaccine uptake.

“There’s unfortunately been increasing anti-vaccine sentiment in the United States,” said Dr. Ericka Hayes at the Children’s Hospital of Philadelphia. “Our recovery is not nearly as quick as we expected it to be and we needed it to be. And again, when you fall below 95% for vaccinations, you lose that herd immunity protection.”

The pertussis vaccine, which also protects against diphtheria and tetanus, is given at two months, four months and six months. Pregnant women can get the vaccine to help protect their newborns, but Hayes said not enough women locally are opting in.

The CDC also recommends adults get follow-up doses every 10 years.

The uptick could also have to do with the type of vaccine being used, writes UCLA epidemiology professor Annette Regan in The Conversation. The old pertussis vaccine, used until the 1990s, used to contain whole, killed B. pertussis bacteria cells. However, Regan explains, those vaccines are more likely to cause a fever or other side effects when babies got a dose.

As a result, we switched to a vaccine that contains parts of the bacteria but not the whole bacteria. These vaccine formulations are believed to cause fewer side effects, Regan writes, but their protection may not last as long.

The Associated Press contributed to this report.