This winter in the San Francisco Bay Area, many children will sit in classrooms and play on the jungle gyms at recess and then go home to attentive parents who work hard to give them every advantage in life. Parents in this part of the country are better educated and wealthier than the average American and can give their children more opportunity. But the Bay Area is also a hotbed of the growing movement to abstain from vaccinations for fear that the shots cause autism and other disorders. Although these parents may have the best of intentions—to protect their kids—they are dangerously misguided.

California is now in the middle of the worst outbreak of pertussis in half a century. This highly contagious disease—known as whooping cough for the distinctive sound its victims make when gasping for air after a fit of paroxysmal coughing—was a scourge of childhood until the advent of an effective vaccine against it in the 1940s, which drastically reduced incidence of the disease. The number of annual cases has been climbing in recent years. Last year, though, the rate of infection rose, once again, to epidemic proportions—7,297 known and suspected cases, a fourfold increase from 2009. Whether those refusing the vaccine have helped fuel the current pertussis epidemic is uncertain, but their decisions have created a public health tinderbox: in some Bay Area schools, 40 percent or more of the kids are not vaccinated, leaving them unprotected against pertussis and other preventable diseases, such as measles.

California is hardly the only state grappling with antivaccine sentiment. Significant numbers of parents across the country are declining standard immunizations for their children. The success of any given vaccine depends on so-called herd immunity, in which a high rate of immunization in a population helps to protect those individuals who are not immune. Herd immunity requires high immunization rates—around 95 percent for highly contagious infections like pertussis and measles. When immunization rates drop below the critical level, disease can strike not only unvaccinated individuals but also vaccinated ones, because all vaccines fail to confer immunity in a certain percentage of people. Parents who opt out are endangering not only their own kids but everybody else’s, too—including those who cannot be vaccinated because they are too young or immunocompromised, as well as youngsters who have received their shots.

Vaccine anxiety has been around for as long as there have been vaccines, but the fear of autism originated with a paper published in the Lancet journal in 1998. On the basis of a study of 12 children, author and British medical doctor Andrew Wakefield claimed to have found a link between the measles-mumps-rubella (MMR) vaccine and an autismlike disorder. Antivaccination groups and the media pounced on the news, and before long the alleged vaccine-autism connection became a Hollywood cause célèbre; former actress and Playboy model Jenny McCarthy claims that MMR caused her son’s autism.

In February 2010 the Lancet retracted Wakefield’s infamous paper. That leaves no scientific evidence to support the assertion that vaccines cause autism or other chronic diseases. Unfortunately, fear is far easier to ignite than to extinguish. Some parents caught in the crossfire between scientists and charlatans have decided, against all reason, that the vaccines are more dangerous than the diseases they protect against.

They are wrong. Vaccines may provoke a low fever and other unpleasant symptoms. Very rarely, side effects are serious, such as an allergic reaction. The risk of unprotected exposure to vaccine-preventable diseases is far higher: for example, more than 90 percent of unvaccinated people exposed to measles will become infected.

Each state has its own immunization requirements for schoolchildren. Yet in 48 states parents may exempt their kids on the basis of religious or philosophical beliefs (only Mississippi and West Virginia disallow exemptions). The right to decide what is best for oneself and one’s children ends where science has so clearly documented a threat to public welfare. It’s time for the other 48 states to eliminate these exemptions and adopt strict enforcement policies to ensure that kids get their jabs. In the interim, doctors need to be patient but firm with fearful parents, explain why vaccines are essential and help restore the public’s faith in science.