A rising number of parents in more than half of states are opting out of school shots for their kids. And in eight states, more than 1 in 20 public school kindergartners do not get all the vaccines required for attendance, an Associated Press analysis found.
That has health officials worried about possible new outbreaks of diseases that were all but stamped out.
The AP analysis found more than half of states have seen at least a slight rise in the rate of exemptions over the past five years. States with the highest exemption rates are in the West and Upper Midwest.
It's "really gotten much worse," said Mary Selecky, secretary of health for Washington state, where 6 percent of public school parents have opted out.
Rules for exemptions vary by state and can include medical, religious or _ in some states _ philosophical reasons.
Parents' reasons for skipping the shots vary. Some doubt that vaccines are essential. Others fear that vaccines carry their own risks. And some find it easier to check a box opting out than to get the shots and required paperwork.
Still others are ambivalent, believing in older vaccines but questioning newer shots against, say, chickenpox.
The number of shots is also giving some parents pause. By the time most children are 6, they will have been stuck with a needle about two dozen times _ with many of those shots given in infancy. The cumulative effect of all those shots has not been studied enough, some parents say.
"Many of the vaccines are unnecessary, and public health officials don't honestly know" the effects of giving so many vaccines to such small children, said Jennifer Margulis, a mother of four and parenting book author in Ashland, Ore., a small liberal community that has unusually high vaccination exemption rates.
But few serious problems have turned up over years of vaccinations, and several studies have shown no link between vaccines and autism, a theory from the 1990s that has been widely discredited.
To be sure, childhood vaccination rates remain high overall, at 90 percent or better for several vaccines, including those for polio, measles, hepatitis B and chickenpox. In many states, exemptions are filed for fewer than 1 percent of children entering school for the first time.
Health officials have not identified an exemption threshold that would likely lead to outbreaks. But they worry when some states have exemption rates climbing beyond 5 percent. The average state exemption rate has been estimated at less than half that.
Even more troubling are pockets in some states where exemption rates are much higher. In some rural counties in northeast Washington, for example, vaccination exemption rates in recent years have been above 20 percent and even as high as 50 percent.
"Vaccine refusers tend to cluster," said Saad Omer, an Emory University epidemiologist who has done extensive research on the issue.
Parents who let their kids skip some vaccines put others at risk, health officials say. Because no vaccine is completely effective, if an outbreak begins in an unvaccinated group of children, a vaccinated child may still be at some risk of getting sick.
Studies have found that measles has suddenly re-emerged in some communities with higher exemption rates. Vaccinated kids are sometimes among the cases, or children too young to be vaccinated.
And measles isn't the only risk. Last year, California had more than 2,100 whooping cough cases, and 10 infants died. Only one had received a first dose of vaccine.
"Your child's risk of getting disease depends on what your neighbors do," Omer said.
While it seems unlikely that diseases like polio and diphtheria could make a comeback in the U.S., immunization expert Dr. Lance Rodewald says it's not impossible.
"Polio can come back. China was polio-free for two decades, and just this year, they were infected from Pakistan. And there is a big outbreak of polio in China now. The same could happen here," Rodewald, of the federal Centers for Disease Control and Prevention, said in an email.
He cited outbreaks of Hib, a disease that can lead to meningitis, among the Amish who don't consistently vaccinate their children. Russia had a huge diphtheria outbreak in the early to mid-1990s, he said, because vaccine coverage declined.
For its review, the AP asked state health departments for kindergarten exemption rates for 2006-07 and 2010-11. The AP also looked at data states had previously reported to the federal government. (Most states do not have data for the current 2011-12 school year.)
Alaska had the highest exemption rate in 2010-11, at nearly 9 percent. Colorado's rate was 7 percent, Minnesota 6.5 percent, Vermont and Washington 6 percent, and Oregon, Michigan and Illinois were close behind.
Mississippi was lowest, at essentially 0 percent.
The AP found 10 states had exemption rate increases over the five years of about 1.5 percentage points or more, a range health officials say is troubling.
Those states, too, were in the West and Midwest _ Alaska, Kansas, Hawaii, Illinois, Michigan, Montana, Oregon, Vermont, Washington and Wisconsin. Arizona saw an increase that put that state in the same ballpark.
Exemption seekers are often middle-class, college-educated white people, but there are often a mix of views and philosophies. Exemption hot spots like Sedona, Ariz., and rural northeast Washington have concentrations of parents who prefer alternative medicine, as well as libertarians who fear giving government too much authority.
Opposition to vaccines "is putting people together that normally would not be together," observed Elizabeth Jacobs, a University of Arizona epidemiologist looking at that state's rising exemption rates.
A national survey of roughly 750 parents, published last month in the journal Pediatrics, found that more than 1 in 10 parents said they refused or delayed shots mainly because of safety concerns.
Many exemption-seeking parents conclude that the dangers posed by vaccine-preventable diseases are less important than the possible harm from vaccines.
"We are being told this by every government official, teacher, doctor that we need vaccines to keep us safe from these diseases. I simply don't believe that to be true. I believe all the diseases in question were up to 90 percent in decline before mass vaccines ever were given," said Sabrina Paulick of Ashland. She's a part-time caregiver for elderly people and mother of a 4-year-old daughter.
"I don't think vaccines are what saved the world from disease," she added. "I think effective sewer systems, nutrition and hand-washing" are the reasons.
Parents say they would like to reserve the right to decide what vaccinations their children should get and when. Health officials reply that vaccinations are recommended at an early age to protect children before they encounter a dangerous infection.
"If you delay, you're putting a child at risk," said Gerri Yett, a nurse who manages Alaska's immunization program.
Analyzing vaccination exemptions is difficult. States collect data differently. Some base their exemption rates on just a small sample of schools _ Alaska, for example _ while others rely on more comprehensive numbers. So the AP worked with researchers at the CDC, which statistically adjusted some states' 2010-11 data for a better comparison.
It's also not clear when an exemption was invoked against all vaccines and when it was used to excuse just one or two shots. CDC officials think the second scenario is more common.
Also, states differ on some of the vaccines required and what's needed to get an exemption: Sometimes only a box on a form needs to be checked, while some states want letters or even signed statements from doctors.
Meanwhile, some parent groups and others have pushed legislators to make exemptions easier or do away with vaccination requirements altogether. The number of states allowing philosophical exemptions grew from 15 to 20 in the last decade.
Some in public health are exasperated by the trend.
"Every time we give them evidence (that vaccines are safe), they come back with a new hypothesis" for why vaccines could be dangerous, said Kacey Ernst, another University of Arizona researcher.
The exemption increases have come during a time when the government has been raising its estimates of how many children have autism and related disorders. Some parents believe the growing roster of recommended shots must somehow be connected.
"I don't understand how other people don't see that these two things are related," said Stacy Allan, a Summit, N.J., mother who filed religious exemptions and stopped vaccinating her three children.
Several parents said that while they believe many health officials mean well, their distrust of the vaccine-making pharmaceutical industry only continues to grow.
"I wouldn't be one to say I am absolutely certain these things are hurting our children," said Michele Pereira, an Ashland mother of two young girls. She is a registered nurse and married to an anesthesiologist. While her daughters have had some vaccinations, they have not had the full recommended schedule.
"I feel like there are enough questions out there that I don't want to take the chance," she said.
Associated Press writer Jeff Barnard in Grants Pass, Ore., contributed to this report.