ATLANTA — More parents are opting out of school shots for their kids. In eight states now, more than 1 in 20 public school kindergartners aren’t getting all the vaccines required for attendance, an Associated Press analysis found.

That growing trend among parents seeking vaccine exemptions has health officials worried about outbreaks of diseases that once 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.

Opt-out reasons vary

Rules for exemptions vary by state and can include medical, religious or — in some states — philosophical reasons.

Reasons for skipping some school shots vary. Some parents are skeptical that vaccines are essential. Others fear vaccines carry their own risks. Some find it easier to check a box opting out than the effort to get the shots and required paperwork schools demand. 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 what the effect of giving so many vaccines to such small children really are,” said Jennifer Margulis, a mother of four and parenting book author in Ashland, Ore.

But few serious problems have turned up over years of vaccinations and several studies have shown no link with 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 even chickenpox. In many states, exemptions are filed for fewer than 1 percent of children entering school for the first time.

Officials: It puts others at risk

Health officials have not identified an exemption threshold that would likely lead to outbreaks. But as they push for 100 percent immunization, they worry when some states have exemption rates climbing over 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 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.

While parents may think it does no harm to others if their kids skip some vaccines, they are in fact putting others at risk, health officials say. 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 communities with higher exemption rates sometimes are places where measles have suddenly re-emerged in outbreaks. Vaccinated kids are sometimes among the cases, or children too young to be vaccinated. 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,” said Omer.

And while it seems unlikely that diseases like polio and diphtheria could ever make a comeback to the U.S., immunization expert Dr. Lance Rodewald with the federal Centers for Disease Control and Prevention says it could happen.

“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 China now. The same could happen here,” Rodewald 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. “Measles is just visible, but it isn’t the only concern,” Rodewald said.

Alaska tops list

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 don’t 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.

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 both alternative medicine-preferring as well as government-fearing libertarians.

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.