© Kevin P. Casey for The New York TimesCelina Yarkin, 44, at her farm with her daughter, Madeline, 6. Ms. Yarkin believes in the benefits of vaccinating children, but many in her community do not.
"You think we're a cut above the rest," said Dr. Maxine Hayes, state health officer for Washington's Department of Health, "but there's something in this culture out West. It's a sort of defiance. A distrust of the government."
The share of kindergartners whose parents opted out of state immunization requirements more than doubled in the decade that ended in 2008, peaking at 7.6 percent in the 2008-9 school year, according to the state's Health Department, raising alarm among public health experts. But last year, the Legislature adopted a law that makes it harder for parents to avoid getting their children vaccinated, by requiring them to get a doctor's signature if they wish to do so. Since then, the opt-out rate has fallen fast, by a quarter, setting an example for other states with easy policies.
For despite efforts to educate the public on the risks of forgoing immunization, more parents are choosing not to have their children vaccinated, especially in states that make it easy to opt out, according to a study published on Thursday in The New England Journal of Medicine.
And while the rate of children whose parents claimed exemptions remains low - slightly over 2 percent of all kindergarten students in 2011, up from just over 1 percent in 2006 - the national increase is "concerning," said Saad Omer, an assistant professor of global health at Emory University who led the study.
Families of unvaccinated children tend to live in close proximity, increasing the risk of a hole in the immunity for an entire area. That can speed the spread of diseases such as measles, which have come back in recent years.
The opt-out rate increased fastest in states like Oregon and Arizona - and Washington, before its law changed - where it was easy to get an exemption. In such states, the rate rose by an average of 13 percent a year from 2006 to 2011, according to the study. In states that made it harder to get an exemption from vaccination, such as Iowa and Alabama, the opt-out rate also rose, but more slowly, by an average of 8 percent a year. Mississippi and West Virginia allow no exemptions.
Vaccines are among the most important achievements of modern medicine. Since the first major types came into broad use in the 1940s, they have drastically reduced deaths from infectious diseases like polio and measles. But the virtual disappearance of these diseases has lulled parents into considering the vaccines against them as less necessary, public health experts say.
"Vaccines are the victims of their own success," said Dr. Paul A. Offit, chief of infectious diseases at Children's Hospital of Philadelphia. "When they work, nothing happens."
A distrust of the medical establishment has also fueled skepticism about vaccines. And while the Internet is a powerful source of information, it has also allowed the rapid spread of false information, such as the theory by Andrew Wakefield, a former British surgeon, that the measles-mumps-rubella vaccine was linked to the onset of autism.
"With the Internet, you can have one cranky corner of Kentucky ending up influencing Indonesia," said Heidi Larson, a lecturer at the Project to Monitor Public Confidence in Immunization, at the London School of Hygiene and Tropical Medicine.
Dr. Omer's study categorizes state exemption policies on a scale from easy to difficult. The easiest rules require parents only to fill out a standardized form, which often involves merely checking a box. More stringent policies require parents to write a letter, detailing precisely why they believe their children should be exempt. "These laws have an impact," he said. "The idea is to nudge the balance of convenience away from getting exemptions."
Parents who refuse vaccines tend to be more educated, and often more affluent than the average, researchers say.
Jonathan Bell, a naturopathic doctor in Washington State who encourages his patients to vaccinate their children. Those who opt out, he said, tend to distrust the public health establishment because of what they see as its unsavory connections with the pharmaceutical industry. "The argument is, 'Oh no, I'm putting off vaccines,' " he said. " 'I'm part of a group that's smart enough to understand the government is a pawn of big pharma.' "
Still, he said that only a small group is adamantly against vaccines, with many of the rest trying to stagger or individualize the schedules of inoculation for their children. Others had opted out simply because it was easy.
A stronghold of vaccine skeptics is Vashon Island, a short ferry ride from Seattle, where the share of parents who have opted out of having their children vaccinated has been as high as one in four. Celina Yarkin, a resident, tries to persuade parents there to immunize their children. This school year, even with the new law, she took to the local school a large sign explaining the benefits of vaccination. She praised the parents' concern for their children, and their determination to decide what was best for them.
"A lot of it is positive," she said. "People just don't just want to take their doctor's word for it. But with vaccines, it just takes this crazy turn."
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