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When Should Vaccines Be Mandated?

IPR economist Charles F. Manski posits methods for decision making under uncertainty

How to decide whether to vaccinate a population when officials know only some of the vaccine's effects?

When faced with an outbreak of a communicable disease, such as the current outbreak of measles that started in California, should federal and state governments require vaccinations to protect the health of their citizens—or should such decisions be voluntary?

Deciding on the right course of action when it comes to vaccines is difficult because officials are working in an environment of “partial knowledge”—that is, they can point to some of a vaccine’s effects, but not all. IPR economist Charles F. Manski addresses this topic in a recent IPR working paper.

Charles F. Manski

Take the case of Ebola, potential vaccines for which are currently undergoing clinical trials. But the trials can only alert the FDA to the vaccines’ effects on those groups who were tested—not the rest of the population.

“The whole contagion process means that if you vaccinate a higher fraction of the population, it not only protects those people, but to some extent, it protects other people as well,” Manski explained, due to the fact that those vaccinated break the chain of disease transmission. This effect is known as “herd immunity.”

Because clinical trials cannot measure herd immunity, however, the FDA does not account for it during the vaccine approval process. Manski set out to address ways for the FDA and others to do just this by providing quantifiable decision-making criteria. In the working paper, he also tackles how states can decide whether to mandate vaccinations once the FDA approves the vaccine.

As evidenced by the real-world example of the recent measles outbreak, there is a point beyond which herd immunity falters, allowing the disease to spread.

“In a situation like that, you can’t just count on people making decisions on their own to achieve a high enough vaccination rate. A state, city, or school system might come in and say, ‘We’re going to have to mandate this,’” Manski said.

Manski is building off of his previous work in a 2010 Proceedings of the National Academy of Sciences article, where he formulated the percentage of a population that would need to be vaccinated in order to contain a disease. But that paper “was in kind of an idealized world. It’s very hard to have a public policy that says, ‘Ninety percent of the people will get vaccinated, and 10 percent will not,’” Manski noted.

The more pragmatic policy concerns addressed in his working paper include decision-making criteria for mandating or not mandating a vaccine based on the partial knowledge an official might have—such as the number of people who choose to get vaccinated after a vaccine is approved, and the illness rate among unvaccinated individuals.

Charles F. Manski is Board of Trustees Professor in Economics and an IPR fellow. Read the working paper, “Vaccine Approvals and Mandates Under Uncertainty: Some Simple Analytics” (WP-14-29).

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