Herd Immunity in the Jet Age

By 2012, I thought we'd already be beyond the jet age. Although, if you go by the GOP debates, determining if a "moon-colony" could apply for U.S. statehood is now our top domestic concern, so maybe we're finally getting beyond the jet age?

An idea central to controlling infectious diseases is herd immunity. This is the idea that vaccinating a proportion of the population (e.g. 80% for mumps or 95% for measles) will protect the entire population, even the unvaccinated. In a paper presented recently at the meeting of the American Mathematical Society and the Mathematical Association of America and discussed in the Economist, Petra Klepac and colleagues wanted to know how increasingly mobile populations with varying vaccination rates would impact optimal vaccination targets for infectious diseases. That is, does it make economic sense to target a herd-immunity threshold? Also, how would high-levels of varicella vaccination in the US vs. low levels in Britain interact to impact chickenpox in both countries?

Dr. Klepac and her team used a susceptible-infected-removed (SIR) mathematical model, which we frequently use in analysis of infection-control interventions. Analysis of their model determined that targeting herd immunity makes sense for an isolated country. However, when international travel was added, she found that a small rate of unvaccinated travelers would reduce the optimal vaccination below the level of herd immunity, so that targeting herd immunity becomes too expensive. Thus, we have to be tolerant of more infections.

There are some other interesting implications of her study, so head on over to the Economist Babbage blog to read more.