Maine residents voting Tuesday in the presidential primary will also have a chance to cast ballots on another issue: vaccine requirements. A statewide referendum asks if voters want to overturn a new law that eliminates religious and philosophical exemptions for childhood vaccines.

Molly Frost of Newcastle wants the new law to remain in place. Her 11-year old son, Asa, has a compromised immune system. He was diagnosed with Hodgkin's lymphoma, a type of cancer, when he was 5, and has relapsed three times. Frost says Asa has undergone several rounds of chemotherapy, radiation and, most recently, a stem cell transplant.

"He at this point has no immunity against any of the things he was vaccinated for in the past, and could get very sick from those diseases were he to catch them," she says.

That worries Frost, especially because her family lives in a coastal county where vaccine exemption rates are at least 9 percent — one of the highest rates in the state. She was glad when the Maine legislature passed the law last year intended to protect kids like her son. It aims to boost immunization rates of kids entering school by eliminating non-medical exemptions. The law will go into effect Sept. 1, 2021 — unless opponents win their repeal effort on Super Tuesday.

"It's a huge infringement on personal freedoms," says Cara Sacks, co-chair of the group that put the repeal on the ballot. "On medical freedom in particular."

The group hoping to repeal any ban on non-medical exemptions includes parents like Angie Kenney who want to keep the philosophical exemption for vaccines. Kenney has used the philosophical exemption to refuse immunizations for her kids — ever since her older daughter had an adverse reaction after receiving the chicken pox vaccine at 18-months-old.

"She could not crawl," Kenney says. "She couldn't walk. She couldn't even feed herself. And this went on for almost a year."

Her daughter was diagnosed with a brain injury called ataxia. It's listed by the Centers for Disease Control and Prevention as an adverse event that's known to occur extremely rarely after chicken pox vaccination; Kenney says she received a payment from the National Vaccine Injury Compensation Program. Her daughter has recovered and is now a teenager. But Kenney also has a 4-year-old and doesn't think the state should force her to get either girl vaccinated. "I am not sacrificing my child for the greater good of the community," she says.

Across Maine, though, physicians and health organizations say the new law is urgently needed to protect public health, because more and more parents are using exemptions.

More than 5% of kindergartners in Maine now have non-medical exemptions — more than double the national average.

That's pushed vaccination rates for many diseases below 95% — the critical threshold to achieve "herd immunity" within a community, and thereby avoid spreading a disease to kids who, like Asa Frost, have compromised immune systems.

Pediatrician Dr. Laura Blaisdell says she has daily conversations with parents about vaccines, but has felt helpless in recent years as she's witnessed immunization rates drop in Maine.

"We have gotten to a point where there are no other solutions," says Blaisdell of the new law eliminating non-medical exemptions. She is now the spokeswoman for the group fighting the repeal effort.

Maine has the nation's second-highest rate of pertussis, a vaccine-preventable disease that's also known as whooping cough.

Blaisdell is also worried about measles — if other states have an outbreak of that highly contagious disease, she says, it could easily travel to Maine through the millions of tourists who visit the state each summer.

"That sort of traffic is exactly the sort of traffic that diseases like measles would just love," she says.

More than $1 million has been spent on the referendum battle. The campaign to preserve Maine's new law received its initial support largely from doctors, nurses and health organizations. In the latest campaign filings, the group got a $500,000 boost from the pharmaceutical companies Pfizer and Merck. The trade group Biotechnology Innovation Organization, which represents the biotech industry, also contributed $98,000.

Meanwhile, the campaign to repeal the law, Yes on 1 for Maine, adopted "Reject Big Pharma" as its primary slogan.

That campaign received much of its early support from individual donations and chiropractors. More recently, the Organic Consumers Association contributed $50,000. The Minnesota-based group has been criticized for stoking vaccine fears and causing a measles outbreak in the state's Somali community three years ago.

The backlash that's erupted over Maine's new law doesn't surprise Alison Buttenheim, a professor at the University of Pennsylvania. She studies vaccine hesitancy and state exemptions. When states eliminate entire categories of exemptions, she says, some people perceive that as parental rights being sacrificed for public health.

"You sort of wonder, could Maine have taken a different policy step?" Buttenheim says, "Maybe, [by] making those exemptions harder to get, [the state could have] accomplished the same goal of coverage and disease protection — without having to go through a big repeal effort."

If the new law is preserved, Maine would join four other states that don't allow any non-medical exemptions for vaccinations.

This story is part of NPR's reporting partnership with Maine Public Radio and Kaiser Health News.

Copyright 2020 Maine Public. To see more, visit Maine Public.