When Sen. David Zuckerman (P/D-Chittenden) was a 13-year-old boy growing up in Massachusetts, his father, a thoracic surgeon, died of stomach cancer.

The son attributes his father's death to the radiation he was exposed to while deploying a new medical procedure to treat cancer patients. Without protection from a now-standard lead vest, at least four physicians suffered the same fate, according to Zuckerman.

Science saves lives, he learned, but it can also take them.

As a state senator and now as the Progressive/Democratic candidate for lieutenant governor, that experience informed his position on childhood vaccinations: Zuckerman doesn't think they should be forced on kids who may have adverse reactions to shots against diseases such as diphtheria, rubella, polio, mumps and whooping cough.

That so-called "philosophical exemption" was an option for Vermont families until this year, after the legislature voted in 2015 to abolish it in the interest of public health.

During 18 years as a state legislator, Zuckerman has defined himself as a champion of outlier causes. The organic farmer was an early supporter of same-sex marriage, of challenging genetically modified foods and of legalizing marijuana. But his stance on vaccinations — and his 2015 vote to preserve a parent's right to choose — might just be the biggest outlier of them all.

His Republican opponent, Randy Brock, cited Zuckerman's vaccination viewpoint as evidence that the pony-tailed Progressive from Hinesburg lacks the judgment to be a heartbeat away from the governor's office.

"He's a strong supporter of laws that would expose our schoolchildren to dangerous diseases by discouraging vaccinations," Brock said in a radio advertisement.

But it's not just polar-political opposites who question Zuckerman's logic. His defense of those who question vaccines has put him at odds with some of the same people who support his other causes. That could cost him votes as he seeks statewide office for the first time.

"I don't agree with the position he takes," said Jack McCullough, a left-leaning activist from Montpelier who belongs to the pro-vaccine group Parents Against Preventable Infections. He'll vote for Zuckerman anyway.

But for another member of Parents Against Preventable Infections, who asked not to be identified, Zuckerman's stand could be a deal breaker. Although she agrees with him on nearly every other issue, "His position about vaccinations definitely turns me off to voting for him," she said. "I think it's important for our political leaders to make policy decisions based on the best science we have available."

Meanwhile, those who want to restore the philosophical exemption have fully embraced Zuckerman. The group Vermont Coalition for Vaccine Choice is promoting his candidacy on its Facebook page, complete with a link to make campaign contributions.

"Zuckerman was one of only a few Vermont senators to stand up in support of parents making these important medical decisions in 2015," wrote one commenter on the group's Facebook page.

"I think David at the time was regarded by many as a hero," said Jennifer Stella of Waitsfield, cofounder of the Vermont Coalition for Vaccine Choice. "He was truly trying to find a way to protect all children."

Stella said her two children had bad reactions to vaccines — one had seizures, the other got full-body eczema — and their pediatrician supported a decision to forgo additional immunizations. With that doctor's note, her children might have qualified for legal "medical exemptions" in Vermont, but Stella exercised her philosophical option instead.

Those who distrust vaccinations do so for a range of reasons that include allegations that they cause autism and the U.S. Centers for Disease Control is part of a conspiracy to cover up the evidence. Those who support them consider such theories a threat to public health. A critical percentage of people have to be vaccinated in order to preserve the "herd immunity" that keeps multiple debilitating diseases at bay. Children with compromised immune systems are particularly vulnerable if their un-immunized classmates carry the illnesses.

"Getting diphtheria, tetanus, or pertussis disease is much riskier than getting DTaP vaccine," the CDC advises on its website.

When lawmakers took up the issue last year, measles and whooping cough were reemerging worldwide. Roughly 89.7 percent of the state's kindergartners were fully immunized, according to the Vermont Department of Health. But at some schools, the rates were dramatically lower: Elmore Elementary School reported 68.4 percent; Windham Elementary, 53 percent. At the Lake Champlain Waldorf School, only 32 percent of the students had been fully immunized.

Medical exemptions and religious objections didn't explain the variations. Many Vermont parents — some of them highly educated — weren't getting their children immunized on the grounds that they were philosophically opposed.

Vermont Health Commissioner Harry Chen led the charge to change the law to eliminate that option. Initially, Gov. Peter Shumlin opposed him. Zuckerman did, too.

After the House voted to strike down the philosophical exemption, Zuckerman cautioned his colleagues in the Senate.

"We are adjusting the laws of our state to such that someone else is determining — we in this room are determining — what is going to go into every single person's body," he argued on the Senate floor.

At the time, Seven Days reported that Zuckerman used the word "disputed" to describe the science of vaccination safety. 

Despite his oratory, the Senate voted 18-11 to eliminate the philosophical exemption as of July 1, 2016. The religious and medical exemptions were left in place.

Days after the debate, pro-vaccine backlash compelled Zuckerman to further explain his vote. "The past 48 hours have been some of the most difficult of my serving in political life," he told colleagues, sharing the story of his father's death.

"Why do I tell you this? Because science is good, but it is not perfect," he said. "Such imperfection can cause harm. Absolutism can cause harm."

Now running for higher office, Zuckerman has retold his family history on the campaign trail, at candidate debates and in online forums. He says his position on vaccinations has always been nuanced: He's not anti-science, he considers vaccines effective in eradicating diseases and his own daughter is receiving the full slate of shots. "When I have the conversation with voters, they learn that the issue is not so bipolar. There's complexity," he said. "They don't walk away with the perception that I'm an anti-vaxxer."

Brock has called Zuckerman out for softening his position: "His vaccination stance seems to vary depending on the political season."

Meanwhile, the Vermont Coalition for Vaccine Choice is trying to restart the debate over immunization safety by showing the film Vaxxed: From Cover-Up to Catastrophe at six locations throughout Vermont, including four the first week of November. Stella said the preelection timing is a coincidence.

Zuckerman said he's open to seeing the film but not to stumping for it. Asked whether he bears any responsibility or credit for fueling the vaccination debate in Vermont, the lieutenant governor hopeful said, "I don't think I created any more fuel than was already there."