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.

On Thursday, the Centers for Disease Control and Prevention said that 14,569 cases of whooping cough had been reported so far in 2024. That’s a significant increase over last year’s total of 3,475 cases.

According to a CDC spokesperson, preliminary cases reported so far this year are the highest since 2014.

The bacterial illness is officially called pertussis but is often referred to as “whooping cough” because of the sound people — especially babies — make when trying to get enough oxygen despite ongoing coughing fits.

Doctors said the newly reported numbers are likely a vast underestimate of the true spread of the highly contagious respiratory infection.

“For every case of whooping cough we find, there’s probably 10 of them out there that didn’t come to medical attention,” said Dr. Jim Conway, a pediatrician and infectious disease expert at UW Health in Madison, Wisconsin.

The new numbers are a sign that whooping cough cases are returning to pre-pandemic levels, when about 10,000 cases were reported per year, nationally, according to the CDC. Growing anti-vaccine sentiments are contributing to the outbreaks, experts say.

“We’ve been seeing increasing amounts of disease occurring in adolescents and the adult population because they’re not getting vaccinated like they should,” Tan said.

This is an age group that may be particularly vulnerable if they haven’t received a whooping cough booster since childhood. Protection from pertussis vaccines doesn't last forever; in fact, boosters are needed every 10 years, according to the CDC.

Some areas of the country are seeing massive spikes. The Wisconsin Department of Health Services, for example, said Thursday that pertussis cases are 10 times higher than they were last year, with 625 confirmed cases in 2024 compared with 51 in 2023.

“While people can get pertussis at any age, Wisconsinites aged 11 to 18 currently make up nearly half of all cases reported so far this year,” the state’s health department said in a news release.

“This is rising really quickly,” Conway said. “That has us concerned.”

Cases are rising, too, in Connecticut. “I don’t think we’ve seen this many in over 15 years,” said Dr. Andrew Carlson, a pediatrician and medical director for primary care at Connecticut Children’s in Hartford. The state has logged 111 whooping cough cases this year so far, compared with just 11 in 2023.

Doctors at Duke University School of Medicine in North Carolina and Children’s National Hospital in Washington, D.C., told NBC News that they hadn’t seen any recent cases of whooping cough. Georgia saw an uptick in whooping cough cases over the summer, said Dr. Andi Shane, division chief of pediatric infectious diseases at Children’s Healthcare of Atlanta, but that’s since declined.

In areas that have been affected, outbreaks have popped up among teens and young adults, including at a Philadelphia-area high school in April. Last weekend, Portland State University canceled a football game following a surge in whooping cough cases among players.

“During the spring, we saw a high proportion of cases in adolescents,” said Dr. Karen Ravin, division chief of infectious diseases at Nemours Children’s Health in Delaware. “The vaccine is effective, but immunity wanes over time, so it’s important for parents to make sure their kids are up to date with their vaccines.”

The 'heartbreaking' side of whooping cough

People with lingering coughs may go to the doctor but be sent home after testing negative for flu or Covid. The whooping cough can spread that bacteria through their coughs for a long time if not treated.

“You can shed this bacteria for three or four weeks,” Conway said. “Once it gets into a community, it’s pretty hard to stamp it out.”

Most older kids and otherwise healthy adults who become infected usually don’t experience much more than that annoying cough.

Doctors worry most about newborn babies who become infected. Their tiny airways can’t handle the coughing fits associated with whooping cough, and they must often be put on ventilators to help them breathe.

“That’s the heartbreaking side of this,” Conway said. “Those babies are coughing so much they can’t eat, they can’t drink, so they end up in the intensive care unit.”

When to get whooping cough vaccines and boosters

Pregnant women are encouraged to get a whooping cough booster during their third trimester. This gives their babies some level of protection when they’re born — when they’re most vulnerable to pertussis — and before they’re eligible for the shots themselves.

Babies should begin their first round of pertussis vaccines at 2 months, with follow-up shots at 4 and 6 months, according to the CDC. That’s the DTaP vaccine, which helps protect against three diseases: pertussis, diphtheria and tetanus.

Another shot is given before the child reaches age 2, and again around the start of kindergarten.

The vaccine works well against diphtheria and tetanus, but is less effective over time for pertussis.

That’s why boosters are recommended about every 10 years, starting in the tween years, as kids start middle school.

“I can’t overemphasize the importance of vaccination,” Shane said. “The way we’re going to make sure that we all have a successful respiratory season is to do everything we can to protect ourselves.”

This story first appeared on NBCNews.com. More from NBC News:

• Fall is approaching, but mosquito season isn't over
• The U.S. health care system is failing, a startling new report finds
• Sean Combs under suicide watch by jailers as he awaits sex trafficking trial