Christopher Stookey, also a member of the co-op, has heard those concerns but decided there isn't enough evidence to warrant skipping shots for his four young children. He pointed out that medical experts two years ago retracted an often-cited 1998 study that ilinked autism with the measles, mumps, and rubella vaccine. (A review of vaccine safety by the National Academies' Institute of Medicine last year discredited any MMR vaccine link to either autism or Type 1 diabetes.)

"My wife and I are supporters of naturopathic medicine," Stookey said, but when it comes to immunization and contagious diseases, they generally take their doctor's recommendations. The Stookeys do, however, ask to delay some of their children's shots "so the immediate impact of several vaccines isn't as great."

Rebecca Capuano, who writes for a group blog at TheHomeSchoolMom.com and homeschools her two girls (ages 4 and 7) in Roanoke, Va., said she has done her own research and concluded there are too many risks associated with vaccination: "That is the case for me while my children are young—I'd say under 12." When her children are older, she'll take a second look at "each one of the vaccines," but in the meantime, Capuano wonders why her babies should be jabbed for diseases they're unlikely to get, like sexually transmitted hepatitis B.

When Steven David Horwich's two children attended a private school in California, their family doctor—who believed there was "no scientific proof" that vaccines were effective—wrote notes to the school saying the children had been immunized, although they had not. Horwich, a playwright, educator, and author of Connect the Thoughts, a curricula series marketed to homeschoolers, said his doctor's medical perspective helped confirm his own research and conviction that vaccination is "more dangerous than helpful, and not a very good idea."

Horwich said his children were less likely to get sick after he began educating them at home—homeschooling was a natural deterrent to germ spread because exposure to other students was limited. His two children are now adults, but Horwich said he's worked with hundreds of homeschoolers over the past decade, and met many who refused inoculation because of their personal research or for religious reasons. Some had an added motive: "They didn't trust anything that the government recommended. ... If the government says do it, they don't."

U.S. health officials have stoked that mistrust in recent years by recommending all preteens—girls and boys—be vaccinated for human papillomavirus (HPV), which spreads only through sexual activity. Social Science & Medicine this summer reported that homeschoolers are significantly less likely than public-school children to have received an HPV vaccine. However, they aren't less likely to have gotten shots for tetanus or meningitis, according to the same study.

Jay Wile, who has written a science textbook series popular among home educators, occasionally speaks at homeschool conferences in defense of vaccines. When he spoke to WORLD last year for a story on whooping cough (see "Risking deadly diseases," Feb. 12, 2011), he told me he was "sympathetic to a lot of folks who don't trust the government, because there are aspects of government I don't trust, either." But immunization isn't one. Wile said vaccines do come with risks of side effects—fevers, seizures, and in very rare cases, severe allergic reactions—but with a disease like measles, "the question is, do I worry about a side effect of the vaccine, or do I worry about someone bringing the virus in from somewhere else? And based on risk medicine analysis, it's a riskier thing to not get the vaccine." Wile keeps a folder with emails from conference attendees who say he changed their mind.