Sanford schools are reporting a pertussis outbreak, with four confirmed cases at Margaret Chase Smith primary school and two cases at Sanford Junior High School.

Pertussis, also known as whooping cough, remains a significant and persistent public health threat in Maine, especially for infants too young to be immunized or for children who can’t get vaccinations because they have childhood diseases such as leukemia.

Maine had the third-highest pertussis rate in the nation – 18.3 per 100,000 residents in 2016, the latest year for which nationwide comparisons are available, according to the U.S. Centers for Disease Control and Prevention.

In a letter sent to parents Wednesday, school officials warned that pertussis is a highly contagious, violent cough that can persist for months, and can be deadly to infants.

An outbreak is defined as three or more cases in one place – such as a school, dormitory or workplace.

Maine has experienced sky-righ pertussis rates for several years, and Maine schools have one of the worst vaccination rates in the country, according to vaccination data released by the Maine Center for Disease Control and Prevention and the federal CDC, in part because Maine makes it relatively easy for parents to opt out of school-required vaccines.

In spring 2017, several schools in the state reported pertussis cases in letters sent home to parents, including Greely in Cumberland, Waynflete in Portland, and schools in Yarmouth and South Portland. Students can be kept out of classes for 21 days if they have an infectious disease.

Margaret Chase Smith primary school’s kindergarten vaccination opt-out rate for nonmedical reasons was 5.3 percent in 2016-17, slightly above the state average of 4.8 percent. The nonmedical opt-outs in Maine for kindergartners are consistently among the highest in the country, according to the U.S. CDC.

Vaccines are overwhelmingly safe and a mountain of research has concluded that there is no link between vaccines and autism. A 1998 British study that claimed a link was withdrawn and retracted.

But rumors that vaccines aren’t safe persist among some parents, and this has led to a return of some childhood diseases, including measles, mumps and chickenpox.

This school year, Maine required pertussis booster shots for students entering seventh grade, and health officials predicted that over time the new shot requirement would help reduce the number of cases. Maine was one of the last states to require the middle school booster for pertussis.

Dr. Siiri Bennett, Maine’s state epidemiologist, said it will take time for the new middle school booster policy to take full effect on infection rates. But she said there’s some encouraging signs so far in 2018, with 63 cases through Monday compared with 133 cases through the same time last year.

“We’re significantly below last year. We’re trying to break the cycle,” Bennett said. About 40 percent of all pertussis cases in 2017 were patients age 11 to 19, so if those rates decline it should help bring overall pertussis rates down, she said.

In 2017, Maine had 410 cases of pertussis, the most since the 557 in 2014, according to the Maine CDC. In 2012, Maine reported 737 pertussis cases. In the late 2000s, there were fewer than 100 cases per year in Maine.

Sanford Superintendent David Theoharides said he was first notified last week by the CDC about a couple of confirmed cases at Margaret Chase Smith School and notified parents of students in the school. When additional cases were confirmed Monday, state health officials asked him to notify all parents in the district.

“Why it happened at Margaret Chase Smith School this year, who knows?” said Theoharides, noting that school officials are “vigilant” about reaching out to families to make sure as many children as possible have their required vaccines.

Sanford school officials recommend that parents of children with a severe or prolonged cough contact their doctor. They also recommended that parents check with their doctor to ensure that their child is up to date on the pertussis vaccine. Booster shots are now recommended for children 11 to 18 years old.

Theoharides said he is glad students are on school vacation next week because it gives custodial staff a chance to do a heavy-duty cleaning of the buildings. He has reached out to the CDC to see whether there are specific recommendations for cleaning in school buildings with confirmed pertussis cases.

Legislation to make it more difficult for parents to opt out of childhood vaccinations in recent years fell short after the Maine Legislature failed to override vetoes by Republican Gov. Paul LePage. One proposal would have required consultation with a medical professional before parents could forgo vaccines for their children on philosophical grounds.