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Vaccine freeriding and public health parasitism

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One thing you’ll learn in almost any Stanford biology course is that nature is brutal in its neutrality. Predators devour the young and sick; creatures parasitize one another, gnawing away from the inside out; natural disasters decimate entire populations that struggled to adapt to their environment for thousands of generations. To say that something is “natural” has no bearing whatsoever on whether it is morally good or bad; cancer and infanticide are as natural as friendship and joy. Looking at nature, however, can help us formulate sound policies because problems that humans face often mimic those in nature.

Humanity is a herd, and we’ve got herd problems. All seven billion of us, flying from one side of the world to the other every day, many packed tens of thousands per square mile or more, constitute a pretty impressive mega-herd of humans. However, unlike most herds in the animal kingdom, we often protect our very young and very old, our sick and our dying. Or at least we try.

But this collective protection is being threatened by an outbreak of vaccine free-riding, a metaphorical epidemic that may cause far more literal ones to ravage vulnerable segments of our population. Human populations protect themselves from many diseases through vaccination. Members of a population are all protected from a disease if enough people are vaccinated that the infection is unable to spread, a phenomenon called “herd immunity.” This phenomenon is important because vaccines cannot be administered to everyone, including young infants, people with strong allergies and chronic illnesses, and people with HIV/AIDS. Even diseases that are not usually fatal can be deadly to these vulnerable groups, so their lives depend on maintaining herd immunity.

But the farther the community is above the “herd immunity threshold,” (the proportion of the population that must be vaccinated to prevent a given disease from spreading) the less incentive each individual has to get vaccinated (or get his or her children vaccinated). Some people who want to avoid the small risks associated with vaccines (or believe the risks are more substantial) begin to free-ride, avoiding vaccination themselves while relying on herd immunity to prevent infection. And if enough people are vaccinated, it’s not impossible that someone might even reduce the risk he or she is exposed to by choosing not to vaccinate. But free-riders spoil things for the people who have no choice, those who cannot be vaccinated and minors whose parents enforce the decision not to vaccinate.

This cannot become a true tragedy of the commons. The more people avoid vaccinations, the more each person is incentivized to vaccinate, because their risk of being exposed to infectious diseases transmitted by unvaccinated individuals goes up. A vaccination boom-and-bust cycle might result, where vaccination goes in and out of fashion as communities go below and above the herd immunity threshold.

Keeping up-to-date with vaccinations is a public service that helps protect those in need, and prevents the massive health care costs associated with treating people for preventable diseases–even ones that aren’t usually life-threatening, like chicken pox.  The people evading preventative measures like vaccination costs taxpayers millions of dollars and foil attempts to permanently eliminate diseases.

We see examples of this phenomenon in nature, often in predator-prey interactions.  For example, hares and lynxes follow a 10-year cycle. As the population of hares rises, the lynxes have more to eat, so their population rises too. But as the hares reach the limits of the resources available to them and are eaten by lynxes, their population begins to fall. As the lynxes run out of food, their population begins to fall, until it becomes low enough that the hares start booming again.

As the disease’s “prey,” the last thing we want is to allow people to stop vaccinating just because they haven’t seen the disease around for a few years.  Diseases will keep coming back if we develop a false sense of security. If we expect to protect our most vulnerable members, we need to remain even more cautious about ensuring that people look beyond their own self-interest.

This effort needs to start at home, but unfortunately, Silicon Valley seems to be slacking. At a Pixar-associated day care center only 43 percent of children are immunized. Many of the parents use the California “personal belief exemption” to opt out of vaccination, putting the young children in these facilities at great risk.

The practice (and non-practice) of vaccination is fraught with other important ethical issues like the appropriate limits on the right to practice one’s religion and the rights of children to be protected from potential harm. But this example demonstrates an important but hidden type of social evil. We need policies that eliminate perverse incentives and disincentivize free-riding. Otherwise, the mundane, selfish shortcuts, like littering and cheating and carelessness, snip away at the foundations of a healthy society. If we keep feeding these diseases, they’ll keep coming back. And we’ll keep losing people to diseases we should have eradicated long ago.

Contact Claire Zabel at czabel@stanford.edu and Joseph (Joey) Zabel at joezabel@stanford.edu