By Evan Simko-Bednarski, CNN

Maine has become the fourth state in the nation to prohibit people from opting out of immunization for religious or philosophical reasons.

Governor Janet Mills (D) signed a bill into law in May removing all non-medical exemptions to vaccination from the books.

“As governor, it is my responsibility to protect the health and safety of all Maine people, and it has become clear that our current laws do not adequately protect against the risks posed to Mainers,” Mills said in a statement shared with CNN.

Mills cited an outbreak of whooping cough in three Maine counties, adding that her state has the worst rate of whooping-cough infection in the nation.

She also acknowledged that the immunization issue was “very emotional” for the people of her state.

“People of good will hold sincere beliefs on both sides of the issue,” she said, “but Maine has a vaccination opt-out rate that is three times higher than the national average for students entering Kindergarten and the state ranks seventh in the country for the rate of non-medical exemptions taken among school-age children.”

The State Senate’s Republican leadership did not immediately respond to requests for comment.

The law will take effect in September 2021. Schoolchildren who claimed a non-medical exemption prior to the law taking effect will be allowed to attend school if their parent or guardian provides a written statement from a healthcare professional indicating they’ve been informed of the risks of refusing immunization.

Medical exemptions to vaccinations will still be granted.

With Mills’ signature, Maine becomes just the fourth state in the nation to rule out religious or philosophical exemptions to immunization. California, West Virginia, and Mississippi also lack non-medical exemptions from school immunization requirements, according to data maintained by the National Conference of State Legislatures.

Maine’s new law comes amid a resurgence of measles across the nation. According to the Center for Disease Control and Prevention, 880 cases of measles were confirmed across 24 states between January 1 and May 17 of this year.

Measles vaccinations are 97% effective, according to the CDC. The disease had previously been declared eliminated in the U.S. in 2000.

Maine recorded its first case of measles since 2017 this month, in a child who had been vaccinated against the disease. Maine’s Center for Disease Control and Prevention said the child has since made a full recovery.

Maine’s CDC reported in April that immunization rates among school-age children was on the decline across the state for most diseases, and was already below the level required for so-called “herd immunity” — the threshold at which enough individuals in a population are immune that disease transmission is unlikely even among the unvaccinated.

The agency reported that roughly half of the state’s kindergarten classes do not meet the herd immunity threshold of 95% vaccination.