Pertussis cases have plummeted in Maine since 2019, when the state had one of the highest per capita rates of infection in the country.

The decline coincided with the onset of the COVID-19 pandemic and the lockdowns, mask mandates and social distancing measures that followed. But, while cases have rebounded somewhat in 2022, infection rates are still far below levels found in Maine before the pandemic, a possible result of improved immunization rates resulting from a law upheld by Maine voters in 2020.

Dr. Nirav Shah, Maine Centers for Disease Control and Prevention director, said the sharp decline is partly the result of new school vaccine mandates that went into effect in the 2021-22 school year, as well as COVID restrictions over the past two years that now have largely been lifted.

“We don’t know yet how much of a contributing factor (the school mandate) is, whether it’s a minority factor or a majority factor,” Shah said. “It will take time to know.”

Pertussis, also known as whooping cough, is a bacterial infection that causes a violent, persistent cough.

In 2019, the last year before the pandemic, Maine reported the second-highest per capita rate of pertussis in the nation, at 28.49 cases per 100,000 population. The national average was 5.67 cases per 100,000. In 2022, Maine is on track to record roughly 7 cases per 100,000.

Maine residents voted in a referendum in March 2020, just before the start of the pandemic, to preserve a new law that eliminates religious and philosophic exemptions for school-required vaccines. For several years prior to the referendum, Maine had one of the highest voluntary opt-out rates for school vaccinations in the nation, and one of the worst pertussis rates.

In 2018 and 2019, pertussis outbreaks occurred at schools in Falmouth, Freeport, Portland, Sanford and Thornton Academy.

Maine has so far recorded 92 cases of pertussis in 2022. That compares to 383 cases for all of 2019, and 446 cases in 2018, when Maine’s pertussis rate was the worst in the nation. An updated state-by-state national comparison for pertussis will be released by the U.S. CDC next year.

During the pandemic years of 2020 and 2021, when masking and gathering restrictions were in full force for much of the time – including day care closures – pertussis cases plummeted in Maine and across the United States. Maine reported 28 cases of pertussis in 2020 and 14 in 2021.

While school was back in session full-time in 2021-22, most schools kept mask mandates in place through March 2022. The 2022-23 school year is the first in Maine since the pandemic began without COVID-19 restrictions that include mask mandates, size limits on gatherings and stricter protocols to keep students out of school when infected with COVID-19.

Dr. Dora Anne Mills, MaineHealth’s chief improvement officer, said the relatively lower rates of pertussis in Maine are welcome. But, she warned, more time needs to pass to see whether higher rates of immunization prevent pertussis from circulating in greater numbers again.

The pertussis vaccine is effective, but not as effective as other childhood vaccinations, such as for measles, rubella and polio, she said.

“I worry about pertussis coming back with a vengeance,” Mill said.