Whooping cough is spreading nationwide at the highest levels since 2014. There have been more than 16,000 cases this year — more than four times as many compared to the same time last year — and two confirmed deaths. And experts are concerned that the outbreak could worsen in the fall and winter months.

“More children are going back to school now, [which leads to] greater exposure,” said Dr. Eric Chow, the chief of epidemiology and immunization at the Seattle and King County public health agency. “We’re coming up on the kind of winter season when people are spending more time indoors with other people.”

The disease is most dangerous to babies: 1 in 3 who get it require hospitalization.

Whooping cough cases are especially high right now on the West Coast.

King County, where Seattle is, has seen more this year than any year since 2015 — “and the year isn’t even over yet,” Chow said. He said the county is still seeing new cases of whooping cough every week.

Why the big outbreak now?

Experts say there are a number of possible explanations for the size of the current outbreak.

Doctors are testing for whooping cough more, so they’re identifying more cases.

It’s possible that the bacterium that causes the disease has mutated.

Also, people got behind on their vaccines during the pandemic, and they haven’t caught up.

“One of the challenges that we have with [the vaccine that protects against whooping cough] is that it is a five-dose series over the course of the first six years of a child’s life, so it does require regular visits to the primary care,” Chow said.

And, Chow said, not everyone can get to the doctor regularly.

But access isn’t the only problem.

“There still is a lot of vaccine hesitancy and anti-vaxers out there that will not vaccinate their kids,” said Dr. Tina Tan, a pediatric infectious disease physician at Northwestern University and the president-elect of the Infectious Disease Society of America.

For the first couple of weeks, whooping cough looks like a mild cold, but then the coughing fits start.

Babies who get it “are going to be whooping when they cough,” Tan said. “And they may cough, cough, cough, cough, cough, and then look like they’re not breathing at all.”

Tan said those pauses in breathing are life-threatening, and a sign that it’s time to go to the hospital.

Whooping cough can also lead to pneumonia and other complications.

But babies can’t get their first dose of the vaccine that protects against whooping cough, also called pertussis, till they’re 2 months old.

“That’s why it’s important for pregnant women to get the pertussis vaccine when they’re pregnant,” Tan said, “so that you can protect your baby for the first two months of life until they’re old enough to be vaccinated themselves.”

Even before the pandemic, only about half of pregnant women got the pertussis vaccine. Now, that number is even lower.

In King County, of the 12 babies who have been seen with whooping cough this year, none of their mothers got the shot during pregnancy.

Vaccine fears, and confidence

Dr. Chow, with Public Health-Seattle & King County, said that’s a missed opportunity.

“Sometimes you require a kind of sit-down conversation with the patient who may be a little bit more hesitant or may have encountered misinformation,” he said, “so it just requires a longer time to build trust and rapport.”

Also, not all obstetrician/gynecologists offer the vaccine in their offices, and some people don’t have the bandwidth to go to a pharmacy for a shot.

At a playground in Seattle’s White Center neighborhood, Kay said she has two kids, ages 12 and 4, and she’s always been hesitant about vaccines.

Kay declined to give her last name because she’s concerned about revealing private medical information.

“With COVID, it even made it even more scary, because everybody started coming out saying, ‘The COVID shot’s not actually good for you,’” Kay said. “So I was like, ‘Maybe the other vaccines are bad too.’ And then I went down the rabbit hole of looking up kids who have supposedly gotten vaccinations and passed away.”

Experts say vaccines are very safe and can prevent diseases that once killed or harmed many babies, children, and adults.

Kay eventually got her 4-year-old daughter the shots required for daycare but nothing else.

“It’s hard for me to get childcare, and it’s just easier for me to say, ‘OK, just give me whatever she needs, just to get her into school,’” Kay said.

So her daughter did get the required whooping cough vaccines — but not any COVID shots, which are not mandated.

Aaron Sittinghorse was at the same playground with his 3-year-old daughter. He said the pandemic had the opposite effect on his thoughts about vaccines.

“It opened up my eyes to how important they are,” he said, “and so now, I’m a believer in vaccines. It’s important, even if it’s not for yourself, but for everybody around you.”

Sittinghorse said he saw on the news that there’s a whooping-cough outbreak right now, and it worries him a little — but not too much, because he and his family are up to date on their vaccines.