Immunization rates tied to cost, social networks

Researchers have found that public immunization rates may be more sensitive to changes in a vaccination’s cost than previously thought, and that social networks can play a vital role in determining the severity of an outbreak.

A Harvard study recently published in Proceedings of the Royal Society B was the first to use epidemiological data to model the spread of voluntary vaccination by imitation within a social network. Imitation is the reliance on anecdotal information from personal contacts to determine whether or not to immunize, the Harvard Gazette reports.

The scientists found that as costs, including the perceived risk of side effects, rise, so does free-riding, when people assume they will be protected by other people's immunity. Free-riding leaves more people unprotected and vulnerable to an outbreak. This does not mean that free-riding necessarily destroys vaccination efforts.

“A population of self-interested people can defeat an epidemic," Daniel I. Rosenbloom, a co-author of the study, told the Harvard Gazette. "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."

Feng Fu, a postdoctoral researcher and co-author of the study, told Medical News Today that overall vaccination rates are fragile.

"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," Fu told Medical News Today.

In a situation where people begin to avoid immunization, including during a vaccine scare, imitation can amplify the effects of an epidemic, causing up to a 14 percent drop in vaccination. That can increase the size of a flu-like epidemic by four times.

The sensitivity question, however, works both ways. If a vaccination’s costs are lowered, voluntary vaccination can increase quite rapidly. While the study modeled influenza, the results are thought to be applicable to a wider variety of diseases.