Pertussis is circulating at sky-high rates in York County, with 97 cases so far this year, including 25 cases of the highly infectious disease in September. Kennebunk Elementary School and The Middle School of the Kennebunks reported pertussis outbreaks shortly after school started in September.

Maine in recent years has consistently recorded one of the worst rates of pertussis – also known as whooping cough – in the country. In 2017, the latest year for which state-by-state comparisons are available from the U.S. Centers for Disease Control and Prevention, Maine’s pertussis rate of 27.7 per 100,000 population was the worst in the nation, and more than five times the national average of 4.9 cases per 100,000. Maine had the third-highest rate in 2016.

Pertussis produces a violent cough, especially in infants and toddlers, that is so severe that the bacterial infection can cause vomiting and exhaustion, according to the federal CDC. The coughing can linger for up to 10 weeks. Pertussis can be treated with antibiotics, but unless caught early, the coughing symptoms can persist even after antibiotics are taken.

Maine had 410 pertussis cases in 2017, according to the Maine Center for Disease Control. The statewide numbers are tracking somewhat lower this year, with 262 cases through Sept. 30, the most recent data available.

But York County’s numbers for 2017 and this year are alarming, said Dr. Laura Blaisdell, a Yarmouth pediatrician and vaccination advocate. Blaisdell said too many parents are opting out of childhood vaccines needed to start school, and the pockets of vaccine resistance in the state are worrisome. Babies too young to get their vaccines and the elderly are especially vulnerable to serious cases, requiring hospitalization or causing death.

York County had 77 cases in 2017, but had much lower numbers from 2013-16, in the 20s, 30s and 40s, according to the Maine CDC. It’s unknown what has caused the spike, but Blaisdell said under-vaccinated communities are a prime suspect.

“I’m nervous that there will be an infant death in Maine,” Blaisdell said. “Pertussis is extremely contagious and it will spread quickly in any community where vaccine rates are low.”

Waldo County in recent years also has experienced much higher rates of pertussis than the statewide average, and the county had 34 cases through Sept. 30.

Emily Spencer, spokeswoman for the Maine CDC, said in an email response to questions that the Maine CDC is seeing a substantial number of cases among infants and children under age 6 in York County, although she didn’t immediately have the age breakdowns by county. Statewide, 88 percent of pertussis cases occurred in children up to age 19.

“The short answer is no, we don’t know. We continue to work with our federal CDC partners to brainstorm, get advice, and ensure we are following the most up-to-date prevention and control recommendations. Pertussis is incredibly infectious,” Spencer said. “We know that in Maine it is relatively easy to opt out of vaccination and unvaccinated communities can increase the risk of pertussis.”

The “Tdap” vaccine at Nasson Health Care in Springvale on Friday. The combination is needed once every 10 years.

Maine law permits parents to forgo vaccines for school-age children by signing a form to opt out on religious or philosophic grounds. A bill that would have required the signature of a physician before opting out garnered support in the Maine Legislature in 2015, but Gov. Paul LePage vetoed the bill and a veto override fell five votes short.

Vaccines are overwhelmingly safe and effective, but myths about immunizations causing autism have persisted since 1998, when a since-retracted study claimed a link between vaccines and autism.

The vaccine-induced immunity wanes over time, so Maine implemented a booster shot requirement for middle school students that started in the 2017-18 school year, although like all other vaccines, parents can opt their children out.

“Maine was one of the last states to implement a 7th grade Tdap (tetanus, diphtheria and pertussis) booster. We’re hopeful that this will begin to reduce the number of pertussis cases in Maine by providing increased protection among this age group who has historically experienced high rates of the disease,” Spencer said.

Katie Hawes, superintendent of RSU 21, which includes schools in Kennebunk, Kennebunkport and Arundel, said the school followed all of the recommended protocols for infectious disease outbreaks, including keeping students who contracted the disease out of school for 21 days and sending out districtwide letters to parents. Hawes said she didn’t immediately recall Friday afternoon how many cases were at each school when the outbreaks occurred in September. The Maine CDC was not able to produce numbers of cases at each school Friday afternoon. An outbreak is defined as three or more cases at one location, such as a school, church, workplace or day care.

There was also a reported outbreak in Arundel, although the Maine CDC didn’t have any further details Friday afternoon. In April, Sanford schools reported six cases of pertussis.

Martin Sabol, medical director at Nasson Health Care, a Springvale health clinic, said adults should check to see if they need a pertussis booster shot. Adults who did not receive a middle school booster shot should get a one-time pertussis booster at the time they are due for their 10-year tetanus shot. That would also help reduce pertussis, because in adults symptoms are much more mild.

“There’s always a lot more undiagnosed cases than what is publicly reported,” Sabol said. “People think they just have a cough that lingers, or have bronchitis, when it’s actually pertussis. And it can spread very easily to very young people.”