Maine continues to experience a high rate of pertussis, with 39 new cases reported in May and a total of 208 cases so far this year, well ahead of the 123 cases in the first five months of 2018, according to a report released Friday by the Maine Center for Disease Control and Prevention.

Pertussis is a persistent public health problem in Maine, as the state has the worst per-capita pertussis rate in the nation. Maine’s pertussis rate of 33.16 per 100,000 in 2018 was more than eight times the national average. Maine had 446 pertussis cases in 2018, but 2019 could eclipse last year, which was the worst year for pertussis since 2012.

Recent outbreaks occurred at Portland schools in May, as well as earlier this year at Falmouth High School and Thornton Academy in Saco.

Schools have been the source of many outbreaks this year, and 51 percent of all cases in 2019 – 106 of the 208 cases – have occurred in those between the ages of 11 and 19.

Meghan May, associate professor of microbiology at the University of New England, said low vaccination rates in Maine are “absolutely” a contributing factor, but there doesn’t seem to be any one reason that pertussis is a problem in comparison to other infectious diseases such as measles.

“It’s a multifaceted problem with several different dynamics,” said May, who has researched pertussis. “We don’t know why it’s pertussis here and not measles.”

Pertussis – also known as whooping cough – is a bacterial infection that causes a violent cough that can trigger vomiting and exhaustion. The elderly and babies too young to be vaccinated are especially vulnerable to serious cases, which may require hospitalization and can lead to death. The cough can persist for up to 10 weeks and is treated with antibiotics.

Maine is especially vulnerable to the return of infectious diseases such as pertussis, public health experts have said, because of the state’s dangerously low levels of immunization rates in schools. Maine has consistently been among the worst in the nation for parents forgoing vaccines for non-medical reasons for students entering kindergarten, according to data from the U.S. Centers for Disease Control and Prevention.

The pertussis vaccine’s effectiveness wanes, which is why pediatricians recommend a booster shot in middle school. Though Maine now requires the booster for incoming seventh-graders, it was among the last states in the nation to add the middle school booster, in 2017-18.

And like all other school-required vaccines, parents can opt out of the pertussis booster or other immunizations for their children by signing a form.

That will change for the 2021-22 school year.

Maine lawmakers passed a bill this spring and Gov. Janet Mills signed into law a measure that, starting in 2021, would eliminate philosophic and religious exemptions to school-required vaccines. Maine had 5.6 percent of parents signing philosophic or religious exemptions to vaccines for children entering kindergarten in 2018-19, according to the Maine CDC. Forty-three elementary schools had 15 percent or higher non-medical opt-out rates, putting those schools at increased risk of infectious disease outbreaks.

Peter Michaud, counsel for the Maine Medical Association, which represents doctors before the Legislature, said over time, the new law will improve vaccination rates and Maine’s pertussis rate should decline.

“We are taking the steps that are available to us to protect Maine’s children,” Michaud said. “In the meantime, parents who have children who need their vaccines shouldn’t wait until the law goes into effect. They can protect their children from contagions now.”