More kids are skipping school shots.

More than one in 20 public school kindergartners aren't getting all the vaccines required for attendance in the eight states with the highest rate of shot skipping. Half of the states across the country have seen a small rise in exemptions over the last few years.

Health officials now worry about the possibility of outbreaks in diseases once thought largely obliterated.

More and more, philosophical reasons--parents who oppose vaccines--have been permitted as grounds for exemption.

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.

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.

Parents who support exemptions argue that they should retain the right to decide what's best for their own children. Many say they aren't convinced that the good vaccinations do outweigh their potential harm.

Colorado is second only to Alaska of states with the most incidents of exemptions, with an exemption rate of 7 percent. Minnesota, Vermont, Washington State, Oregon, Michigan and Illinois round out the top eight.

Mississippi had the fewest exemptions in the country.