CAMBRIDGE, Mass., July 29, 2010 -- Public immunization efforts may be much more sensitive than previously realized to small changes in the perceived costs or risks of vaccination, scientists at Harvard University report this week. In some cases, the spread of vaccine avoidance via social networks can make the difference between a minor, localized outbreak and an epidemic four times as large.
The finding, published in Proceedings of the Royal Society B, comes amidst one of the worst pertussis outbreaks in 50 years, in which 1,500 Californians have contracted whooping cough. Public health officials have cited reduced vigilance in vaccinating against the disease, which sickens 90 percent of those exposed to it.
"People sometimes say that voluntary vaccination is doomed to fail because of the 'free-rider' problem, in which people assume they will be protected by other people's immunity," says co-author Daniel I. Rosenbloom, a graduate student in Harvard's Program for Evolutionary Dynamics, led by Martin A. Nowak. "We find that's not true, as a population of self-interested people can defeat an epidemic. But the trouble is, success is sensitive to small changes in perception of a vaccine's costs -- in terms of money, time, inconvenience, or perceived side effects."
Together with lead author Feng Fu, Rosenbloom and Nowak are the first researchers to incorporate epidemiological data into modeling of how vaccination spreads by imitation in a social network. They found that increasing vaccination cost prompts more free-riding and leads to larger epidemics.
"Herd immunity in a social network is fragile," says Fu, a postdoctoral researcher in mathematical biology at Harvard. "As public perceptions of vaccine side effects change, a population can rapidly switch from high vaccination and herd immunity to low vaccination and a larger epidemic."
The good news, the authors say, is that the sensitivity of vaccination to perce
|Contact: Steve Bradt|