Next school year, Maine will start requiring seventh-grade students to receive a second dose of a vaccine that protects against diphtheria, tetanus and pertussis.

The rule was adopted last week by the state Department of Health and Human Services and will go into effect for public school students in September.

Maine has lagged behind most other states in requiring the second dose of Tdap, as the vaccine is called. The state will become the 48th in the nation to require the middle school booster shot. Only Hawaii and South Dakota do not require a middle school dose of the vaccine, according to the Immunization Action Coalition, a nonprofit organization at the University of Minnesota that works to increase immunization rates and educate the public about the benefits of immunization.

Public health experts say improved vaccination coverage for pertussis, a highly contagious respiratory disease also known as whooping cough, should help Maine reduce the number of cases.

The state has one of the highest rates of pertussis in the country, double the national average or higher, depending on the year.

Maine had 21.1 cases of pertussis per 100,000 people in 2015, compared with the national average of 10.3. Over the past several years, the state has consistently ranked far above the national average in pertussis cases, although the numbers vary widely by year. Maine saw a surge of pertussis cases to 737 in 2012 and experienced another spike in 2014 with 557 cases.

There were 281 cases reported in 2015, and through November of last year Maine had 219 pertussis cases.

Martin Sabol, vice chairman of the Maine Immunization Coalition, a group that lobbies for improved vaccination coverage in Maine, said he believes there’s a connection between the high pertussis rates and the lack of required dose for middle school. According to a study published in the December issue of the Pediatrics scholarly journal, vaccination coverage improved by 22 percent in states that adopted a middle school requirement for the Tdap vaccine.

“That’s probably a big part of the pertussis problem,” Sabol said of the state’s failure to require the vaccine for seventh-graders.

Pertussis can be a dangerous disease for infants, the elderly or the immune-compromised. Sabol said infants, who are too young to get the vaccine, are extremely susceptible to the bacteria that cause pertussis, which is “highly contagious.”

“All it takes is someone coughing in the same room for an infant to contract pertussis,” said Sabol, health services director at Nasson Health Care in Sanford.

Dawn Gray, director of the school-based health center for Nasson, said having a school requirement for the second dose will be helpful in improving vaccination coverage because many parents forget about vaccines when their children approach adolescence.

“In the adolescent age, they tend to fall off of getting their well-child visits, and a lot of parents don’t realize their kids still need to get their shots,” Gray said.

Sabol said public health advocates are pleased to see the state require the second dose of Tdap, and in future years will be looking to add more vaccines to the school requirement list, including meningococcal, HPV and hepatitis B. Maine is one of only four states that do not require the hepatitis B vaccine for entry to school.

Dr. Dora Anne Mills, former director of the Maine Center for Disease Control and Prevention, said Maine has a relatively high number of parents who are nervous about the safety of vaccines, so health officials tend to weigh the benefits of adding a vaccine against the potential that the public will resist the new requirement.

Even though research has proved that vaccines are overwhelmingly safe and protect the public from many dangerous diseases, some people believe vaccines can be harmful or are linked to autism. Numerous studies have debunked the theory that vaccines can cause autism, and the 1998 study claiming a link to autism was retracted by the scholarly journal that published it.

Nevertheless, Mills said public health officials have to consider whether Mainers will accept an additional vaccine requirement.

“Every time you expand the required vaccines for school, there’s a backlash,” said Mills, vice president of clinical affairs at the University of New England.

Under Maine law, it is relatively easy for parents to opt out of required immunizations for their children on religious or philosophical grounds – simply by signing a form. As a result, the state consistently has one of the highest vaccine opt-out rates in the nation for children entering kindergarten, according to the U.S. Centers for Disease Control and Prevention.

In the 2014-2015 school year, Maine experienced a record number of chickenpox outbreaks.

Efforts to require parents to do more to opt out of required vaccines – such as a bill that would have mandated that parents consult with a health professional before forgoing vaccines for their children – failed in 2015. Gov. Paul LePage vetoed the measure, and the Legislature did not have enough votes to override the veto.

Sabol said he doesn’t believe pro-vaccine forces will bring the bill back for the 2017 session and may wait to do so until after a new governor is elected in 2018.