Maine has reached “herd immunity” for school-required vaccination coverage for the first time since 2011 – the Maine Centers for Disease Control and Prevention reported on Tuesday – driven by a law that went into effect in 2021 that eliminated philosophic and religious exemptions to attend K-12 schools.

Herd immunity is achieved when at least 95% of a population is immunized against infectious diseases.

“Maine has become a leader in childhood vaccination,” Dr. Puthiery Va, director of the Maine CDC, said in a prepared statement. “It couldn’t come at a better time, as the United States has already seen more measles cases in the first three months of 2024 than in all of 2023. This alarming trend highlights the importance of childhood vaccinations, which reduce the risk that Maine’s youngest residents could face from these harmful and potentially fatal diseases.”

The U.S. CDC is reporting 125 cases of measles in 17 states (none in Maine) so far in 2024.

State lawmakers and the Mills administration prioritized improving Maine’s vaccination rates after years of high rates of religious and philosophic opt outs left the state vulnerable to outbreaks of infectious diseases.

Lawmakers passed the law in 2019, and it survived a people’s veto attempt that would have overturned the law before it was implemented. The new vaccine law went into effect in the 2021-22 school year.


Since then, as families have complied with the new requirements, vaccination coverage in schools has improved, plummeting from 4.5% opting out in 2020-21 – the last year before the law was implemented – to 0.8% in 2022-23 and 0.9% in 2023-24.

During the past two school years, medical exemptions were permitted, but religious and philosophic exemptions are no longer allowed.

Meanwhile, immunizations for all different types of vaccines – including measles, mumps and rubella, pertussis, polio and other vaccines – surpassed 95% of students getting their shots for the first time since 2011, the Maine CDC said.

Herd immunity is the scientific term for when vaccination coverage is so comprehensive among a population that infectious diseases have few opportunities to spread. Public health experts say herd immunity is important to prevent highly contagious diseases from infecting populations.

“Achieving herd immunity among schoolchildren represents a pivotal success for Maine,” Jeanne Lambrew, Maine’s health and human services commissioner, said in a statement.

The percentage of students who received their immunizations for individual vaccines can vary from the opt out percentages for a number of reasons, primarily for missing vaccination records.

For instance, a parent may not opt their student out of immunizations, but may be missing shot records for their student for individual vaccines. Eventually, parents must provide the data to the school or have a valid reason for a medical exemption, but the records may be missing at the time the schools report their data to the Maine CDC.

The Maine CDC will count missing records for individual vaccines as the student being unvaccinated for a particular shot. But because philosophic and religious exemptions are no longer allowed, vaccination coverage overall has improved, even though some records may be missing.

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