Monitoring measles key to predicting epidemics

The dramatic changes in epidemic patterns that occur in large cities can be predicted using a remarkably simple mathematical model, says David Earn, a professor of applied mathematics at McMaster University, Hamilton, Ont. (Science 2000;287:667-70).

After studying 20th-century measles epidemics in London, Liverpool, New York and Baltimore, Earn and collaborators from Cambridge University and the University of Florida concluded that changes in epidemic patterns previously thought to be "chaotic" or "noise-driven," in the mathematical sense, could be predicted using just birth and vaccination data.

"Measles epidemics range from similar outbreaks every year [annual epidemics], to large or small outbreaks in alternate years [biennial epidemics], to very irregular outbreaks of varying size [irregular epidemics]," Earn says. "In some places there are also records of 3-year cycles."

During the past century, epidemic patterns have changed noticeably. For example, some annual epidemics have shifted to biennial, and biennial epidemics have become irregular.

Earn believes that his paper is the first to indicate the influence of changing birth rates in such transitions.

"The research reported in the current paper allows us to explain transitions in epidemic patterns that have occurred in the past, and to predict transitions in the future. These transitions were not previously thought to be predictable. Since we have revealed a certain type of predictability about the epidemic patterns, we have renewed hope that it may be possible to design better vaccination strategies - strategies that are more likely to lead to eradication of diseases such as measles," Earn says. The new mathematical model is applicable to other diseases with short latency and infectious periods, he told CMAJ. "The approach could be used for diseases such as mumps, rubella, chickenpox and whooping cough. It would not apply to influenza or HIV."

However, Earn's elegant model is by no means the last word on the subject, says Sir Robert May, chief scientific adviser to the UK government, in a commentary in the same issue of Science.

"Much relevant work remains to be done in teasing apart the social, genetic, age-related, and other complications that are smoothed out in the usual mass-action assumption," May says.