Maine slipped from 16th to 20th in an annual “healthiest state” survey, a slide that health experts attribute in part to increases in infant mortality and whooping cough, and a decline in childhood immunizations.

America’s Health Rankings, funded by the nonprofit United Health Foundation, annually compiles 30 measurements for each state, including the percentage who have health insurance, infectious disease rates, binge drinking, physical inactivity, violent crime, children living in poverty and the number of dentists per capita.. The Minnesota-based foundation’s mission is to improve Americans’ health and their access to health care, according to its website.

Maine fared the worst among the New England states, and placing 20th matches the state’s worst performance since America’s Health Rankings began in 1990. The state typically has ranked between 10th and 20th. New England states tend to be near the top of list, with Massachusetts, Connecticut and Vermont in the top five. Hawaii garnered top billing this year.

Andrew Coburn, a University of Southern Maine public health professor and associate dean at the Muskie School of Public Service, said there’s no dramatic shift in the state’s health that can explain Maine’s slide in the rankings.

“I don’t think there’s one smoking gun here we can point to as ‘this is the reason why,’ ” Coburn said.

But Coburn said one factor is a reduction in the number of children ages 19 to 35 months who have received all of their recommended vaccinations, which declined from 72.6 percent in 2013 to 68 percent in 2014. Maine’s ranking for childhood vaccinations plummeted from 12th to 35th as a result.

Meanwhile, the rate of whooping cough – known as pertussis – also has climbed in Maine.

“There’s no question some parents have unfounded concerns about immunizations,” Coburn said. “It’s having an effect.”

Fears about autism and that vaccines are unsafe persist despite a mountain of scientific evidence proving vaccines are safe and effective, public health experts have said.

Although it’s not measured as part of America’s Health Rankings, Maine’s rate of unvaccinated children entering kindergarten also has jumped, and is now the fifth-highest in the nation, according to the U.S. Centers for Disease Control and Prevention. Most of the children who are not immunized don’t get vaccinated because parents opt out for philosophical reasons, according to state data.

And pertussis rates in Maine continue to increase, which can be at least partially attributed to parents choosing not to vaccinate, Coburn said. America’s Health Rankings only used pertussis data through 2011, but the numbers mirror the more recent trend of increasing pertussis cases, state statistics show.

The state experienced a pertussis outbreak in 2012 with 737 cases, according to the Maine Center for Disease Control and Prevention. Pertussis cases declined in 2013 to 332, but have jumped again this year to 453 through Dec. 10. All three years show historically high pertussis infections for Maine, rates not seen since at least the 1960s, health officials have said. Several years ago, there were 200 or fewer pertussis infections per year.

Coburn said public health leaders recognize the threat and are on a “full-court press” to reverse the trend of immunization opt-outs. There has been some success with teenagers, as immunization rates for 13- to 17-year-olds improved, according to the rankings.

Meanwhile, the increase in infant mortality is puzzling to Coburn and Deborah Deatrick, senior vice president of community health at MaineHealth.

“It’s a very troubling statistic – infants who die in the first year of life,” Deatrick said. “We need to do a deeper dive into the numbers to find out why.”

Infant deaths per 1,000 live births increased from 5.5 in 2013 to 6.6 in this year’s report, and are much higher than in the late 1990s to the mid-2000s. Meanwhile, other measures that normally go hand-in-hand with infant mortality rates being higher – such as low birth weight and premature births – continued to be much better than the national averages.

“It seems to be contradictory to what we would think would happen,” said Tim Cowan, director of the health index initiative at MaineHealth. Cowan also noted that it’s disturbing that as infant mortality rates are improving nationally, Maine is performing worse.

Maine does have some bright spots, Deatrick said, including a decline in smoking, a drop in cardiovascular disease and residents who are more physically active than the average American. MaineHealth has expanded its “Let’s Go” program, which works with schools to encourage children to exercise, spend less time in front of screens and eat healthy food.

Mainers eat their vegetables better than a typical U.S. resident, consuming two servings per day, according to the rankings. Deatrick said the “eat local” movement may be contributing to Mainers’ newfound love of vegetables. In 2013, Mainers ate only one serving of vegetables per day.

“We are making some progress on the good side of the ledger,” she said.