Quantum: Counting H1N1

A QUANTUM OF SCIENCE

First let's tackle the jargon.

Prevalence is the total number of cases of a disease or infection in a given population. The important thing to remember about prevalence is that it is all the cases, not just the new ones, so the longer it takes to recover from a particular infection, the higher the prevalence will be. Most of the H1N1 figures you hear or read about are prevalence numbers, and as such are not as accurate a reflection as they might be.

Incidence is the rate at which new cases occur in a population, and is a better measure of how fast a disease is spreading because it takes out the duration-of-illness factor included in prevalence. Incidence is usually reported per capita ("3 new infections per 1000 people") or even with a time-element ("14 per 1000 persons-years"). The latter is useful because it lets researchers compare the instantaneous rate of a disease’s spread even in disparate time periods (for example, an incidence of 14 per 1000 persons-years could mean 14 cases would be expected for 1000 persons observed for 1 year, or 50 persons observed for 20 years). The only caveat to this use of incidence is the assumption that the rate will always be linear over the period in question, so longer time periods are more susceptible to incidence errors.

Finally there is the reproduction ratio, which is a measure of how many people each infected person will spread the disease to before they recover. In order for a disease to spread at all that ratio has to be greater than 1 (such that each infected person infects at least one more person). Up to now the ratios estimated for H1N1 Swine flu have been between 1.4 and 1.6. Recently, however, researchers at the National Institute of Health and Medical Research in Paris have re-estimated the ratio using some different assumptions and found it to be between 2.2 and 3.1 in Mexico, well within the range of numbers required for a pandemic. With the recent report of 135 new cases of H1N1 in Japan, the World Health Organization still denies an official pandemic is underway but evidence is mounting that H1N1 may have a higher incidence than previously thought.

For more information:
http://blogs.sciencemag.org/scienceinsider/2009/05/swine-flus-rate.html

http://blogs.sciencemag.org/scienceinsider/2009/05/h1n1-rocks-japa.html

http://www.eurosurveillance.org/ViewArticle.aspx?ArticleId=19205

© A Quantum of Science / Peter Smalley (2009)
Reproduction with attribution is appreciation