As more and more skeptical parents refuse to immunize their children against pertussis (whooping cough), rates of infection can be expected to rise. Glanz and colleagues conducted a case-control study of 751 children (156 with pertussis and 595 controls) enrolled in the Kaiser Permanente of Colorado managed care health plan between 1996 and 2007. They found that unvaccinated children were 23 times more likely to have pertussis than were vaccinated children; 11% of cases were attributed to vaccine refusal.
The study illustrates a larger trend of parents refusing some or all vaccinations for their children, citing safety concerns. The number of refusers, though relatively small, has been growing over the past decade. "Parents today don't think they have to worry about vaccinating," said Lynn A. Slepski, a U.S. Public Health Service officer and former school nurse. "They think it's somebody else's problem. And because they don't see diseases like rubella, measles, and pertussis in the community, they don't understand what the complications can be."
What these parents are relying on, knowingly or not, is herd immunity. "If enough people are immune, a virus doesn't have the opportunity to take hold," said Daniel A. Salmon, a vaccine safety specialist at the National Vaccine Program Office and coauthor of the pertussis study. But herd immunity is no ironclad protection. "It depends on how easily the disease is transmitted, among other factors," Salmon said. "And it isn't the same everywhere. If a community has high rates of exemption,"-a substantial number of children who haven't been immunized-"outbreaks will be worse."
Such is the case in Wales, where a major measles outbreak-now affecting nearly 300 people-has been tied directly to parents' refusal of the measles, mumps, rubella (MMR) vaccine. Wales's National Public Health Service estimates that up to 45,000 schoolchildren haven't been vaccinated and are at risk for contracting measles; 34 people had been hospitalized by early June. And we can expect to see higher measles rates in the United States as exemptions increase. In fact, 11 cases of measles were reported in Brooklyn, New York, in July. "Measles and pertussis are two of the diseases you're going to see first when we have lots of vaccine refusal," said Salmon. "Measles because it's so easily transmitted, and pertussis because it's still out there in the community."
Countering fears about vaccines, especially MMR, which used to be made with the mercury-containing preservative thimerosal and falsely linked to development of autism, is no easy task. Said Slepski, "For every Internet horror story, there are thousands of children who've had their full schedule of vaccines with no untoward effect. It's about helping parents compare the benefits and risks, and the importance of making the right choices."
And Salmon said that takes "someone who is trusted, who can counter their concerns with good science. Because nurses often administer the vaccines, they usually spend more time with parents. They're in the best position to answer parents' questions."
Laura Wallis