Freeport High School is dealing with an outbreak of pertussis, a highly contagious preventable disease, with three cases over the past two weeks.

The Maine Center for Disease Control and Prevention on Wednesday confirmed the outbreak, which is defined as three or more cases of an infectious disease at one location in a close time frame. Maine has the highest rate of pertussis in the country, and is reporting 337 cases through October of this year.

The state recorded 446 cases in 2018, and Maine’s pertussis rate of 33.16 cases per 100,000 was more than eight times the national average.

So far in 2019, 48 percent of all pertussis cases occurred in children ages 11-18, the Maine CDC said.

“We care deeply about student health and are monitoring our students closely for symptoms while at school,” Becky Foley, superintendent for RSU 5, said in a letter to parents. “Pertussis is an infection that affects the airways, and spreads easily from person to person by coughing or sneezing.”

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

Other pertussis outbreaks in 2019 occurred in Portland, Falmouth and Saco schools.

Maine’s low vaccination rates are a contributing factor for why Maine experiences high rates of pertussis, researchers have said. Also, the vaccine has waning effectiveness, which is why a middle school booster shot is recommended.

Maine was one of the last states in the nation to add the booster as a middle school requirement, starting in the 2017-18 school year. So about half of the current high school population would not have been required to get a pertussis booster shot in middle school.

Maine has one of the highest voluntary opt-out rates for school-required vaccines in the nation, and in 2018-19, the latest year data was available, the voluntary opt-out rate for those entering kindergarten was 5.6 percent. Maine lawmakers approved a law that, starting in 2021-22, would eliminate all non-medical opt-outs for school-required vaccines, getting rid of philosophic and religious exemptions. A group opposed to the new law has collected enough voter signatures to place a proposal to overturn it on the March 2020 ballot.