Friday, 03.28.08

The Poking Cure

Photo by Flickr User Blind Grasshopper under a Creative Commons license.

A recent measles outbreak in San Diego highlights the growing number of parents who refuse to vaccinate their children against deadly diseases.

There's some nostalgia among doctors for the good old days when patients did what they were told, and doctors decided what to tell them. But it's hard for the rest of us to mourn this change; in general, patients probably know what they want better than their doctors. Increasingly, however, people are electing to make decisions that affect the whole community, not just themselves -- particularly in the area of childhood vaccinations. As the number of unvaccinated children grows, their right to make decisions about their own child's health is turning into the "right" to bring disease and death to the community.

Contrary to popular belief, vaccines do not work by protecting only vaccinated people. In a small percentage of cases, the vaccine doesn't stimulate significant immune system response, and in most people the immunity wanes over time. Vaccination works by denying the disease "reservoirs" of unexposed people that give infection a toehold from which to attack the broader population. Buildings full of young children with a dubious respect for personal hygiene are a perfect place for disease to thrive; that's why childhood vaccinations have been such a massive public health success. The broad concept is "herd immunity." The object is not to make everyone immune, but to make a total lack of immunity very rare. That way, even if it manages to infect someone, the disease probably won't encounter another victim. Herd immunity wiped out smallpox, virtually vanquished polio, and could eventually push other diseases into extinction.

That is, if we keep it. It doesn't take many unvaccinated individuals to destroy herd immunity, which breaks down when the rate of non-vaccination reaches somewhere between 5% (whooping cough and measles) and 20% (rubella). These days, a host of health worries are causing more and more parents to keep their children unvaccinated, or partially vaccinated, due to worries that the vaccine causes autism, or that their child will be one of the very few whose systems respond badly to the vaccine. In states that let parents opt out of vaccination, 2.5% opted out in 2004. But because parents who opt out tend to cluster -- ironically in affluent, educated areas -- and in some communities, the percentage is already high enough to erode herd immunity. In 1976, the United States had 1,010 cases of whooping cough. By 2003, that figure was 11,647. Between 2001 and 2003, the disease caused 56 deaths, mostly among unvaccinated infants.

Like many public health efforts, vaccination is extremely vulnerable to free riding. As long as 95% of parents vaccinate their children, you are indeed better off not vaccinating yours -- but if everyone does that, many more children, possibly including yours, will die or be permanently crippled. The benefits of vaccination are destroyed by any large number of defectors, which is why exemptions are best reserved for people with immune disorders or other health problems. When it comes to stopping killer epidemics, we'll all be better off requiring people to follow doctor's orders.

— Megan McArdle

Filed Under: health, medicine, society, vaccine

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