Kelly Marshall’s son was just a baby five years ago when news coverage about a feared link between vaccines and autism had reached a fevered pitch.

The Bath woman was on the fence about whether to immunize little Miles. She researched the issue, found no substantial evidence that vaccines could cause autism, and thought back to her own healthy childhood as a fully vaccinated kid.

“We’ve been on time with every recommended vaccine since my son was an infant,” Marshall said.

A small study published in 1998 that stoked fears about childhood vaccines raising the risk of autism has been resoundingly debunked, and the British medical journal that published the study later retracted it.

But some parents continue to worry about the safety of vaccines, despite scientific evidence to the contrary. Some health experts working to boost immunity rates are changing their pitch in response.

New federal data show the percentage of Maine parents choosing to skip their children’s vaccines is on the rise. More than 750 Maine kindergartners started public school in 2013 without receiving all of the required immunizations because their parents opted out, according to the U.S. Centers for Disease Control and Prevention. They accounted for 5.2 percent of all kindergartners in the state, a jump from 3.9 percent in the 2012-2013 school year.

Maine’s rate was the fourth-highest in the nation. Only Idaho, Michigan and Oregon reported higher rates of parents shielding their kids from shots for nonmedical reasons. Maine recorded the largest year-over-year jump of any state in voluntary exemptions.

Without an exemption, kindergarten students in Maine must be vaccinated against pertussis, diphtheria, tetanus, measles, mumps, rubella, polio and chickenpox.

Parents who received exemptions may not have skipped all of the required vaccinations for their children. An exemption could reflect, for example, a child who was vaccinated against measles but skipped a shot for chickenpox.

While the vast majority of Maine’s kindergartners remain vaccinated, even a small breakdown in immunizations can raise the risk of a disease outbreak. “Herd immunity” — when enough people are vaccinated to prevent the spread of infectious disease, even to unvaccinated individuals — begins to falter if 90 percent to 95 percent or less of the population is immunized, health officials say.

While some Maine parents turned down vaccination for religious reasons, the overwhelming majority cited philosophical objections. Maine is among 18 states that allow parents to exempt their children from school-required immunizations for philosophical reasons.

Of the 852 vaccine exemptions in Maine last year, philosophical objections accounted for 766, according to the CDC data. Parents cited religious reasons in 30 cases and medical reasons in 56 instances.

Some parents are hesitant to see their infants and young children stuck with needles numerous times, and say not enough research has been done on the cumulative health effects of so many shots. While some choose to spread their children’s immunizations out over a longer period, medical professionals say vaccines are purposefully timed to be administered when children are at most risk for certain diseases.

“Having seen kids in Maine for the last 35 years, this is much more of a problem than it was 10 to 15 years ago,” said Dr. Lawrence Losey, a Brunswick pediatrician who sits on the state’s immunization task force.

Maine’s approaching the tipping point at which measles, a highly contagious viral illness, could re-emerge, he said. Other states with high opt-out rates, including California, have seen outbreaks of the illness.

For most public health events, Americans with less income and education face statistically higher risk, Losey said. But the reverse is true with vaccine refusal — with educated, well-off parents more likely to skip immunizations for their children, he said.

In Maine, “the smell of salt water puts you at risk,” he quipped.

People with like-minded beliefs about vaccination often cluster, leaving those who skip vaccination more susceptible to outbreaks, health experts say.

A “shocking” study published in March in the journal Pediatrics suggested that public health campaigns promoting vaccines and debunking the supposed links with autism and other health risks have backfired, Losey said. Parents actually were less likely to say they planned to vaccinate their children in the face of such campaigns, which included depictions of brain damage in children with measles.

“We’re trying to change our approach now to draw out from parents, ‘What are your objections, what are your concerns?’” Losey said.

Marshall plans to fully vaccinate her second child, 3-month-old daughter Emily, who’s still too young to be fully immunized against some preventable illnesses, such as whooping cough. Everyone else in the family has been immunized to protect her, Marshall said.

“I feel even more strongly now that it’s the appropriate thing to do,” she said of vaccination.

Whooping cough, or pertussis, is dangerous to babies — who don’t typically exhibit the characteristic “whooping” sound as they gasp for air — because it can rob them of oxygen, leading to brain damage and even death. Children too young for the shots or who haven’t built up enough immunity often catch the disease from a loved one who was never vaccinated or failed to stay current with booster shots.

Several cases of whooping cough were confirmed this week at an elementary school in the central Maine town of Anson. Maine has recorded 346 cases of whooping cough this year, already topping 2013’s total of 332 cases, according to the Maine CDC.

To promote access to immunization, health insurers in Maine and the federal government contribute to a fund administered by the Maine Vaccine Board that pays for free vaccines for all children up to age 19. The vaccines are distributed to health care providers at no cost, but parents may have to pay an administrative fee.

Vaccination Coverage Among Children in Kindergarten — United States, 2013–14 School Year