Good question. A recent paper in PLoS ONE looks at H1N1 in foreign travelers in order to estimate the incidence of this virus in Mexico.
When a new disease comes on the scene, it is easy to underestimate how widespread it is, and to overestimate its severity. This is because at first the sickest people are noticed by the system. SO, everybody known with the disease is sicker, on average, than everybody who actually has the disease, and the extrapolation from known clinical cases to the general population is likely to be a low-ball estimate.
In this paper, the researchers report an interesting method of assessing the severity and distribution of the novel “swine flu” H1N1. The look specifically at travellers from the US, the UK, Spain and Canada to Mexico. These people are more likely to be checked and properly assessed for flu, and I suspect they may overstate their likelihood of having the flu for various reasons. So, one can assume that a much closer to 100% rate of observation would occur with these travelers than with people sitting at home in any given population.
The researchers, looking specifically at April 2009, conclude:
We find that the number of cases in Mexican residents may exceed the number of confirmed cases by two to three orders of magnitude. While the extent of disease spread is greater than previously appreciated, our estimate suggests that severe disease is uncommon since the total number of cases is likely to be much larger than those of confirmed cases.
The methods used in this paper are beyond what I’m prepared to discuss and evaluate. But you can evaluate them directly by reading the paper, which you can access by clicking here. PLoS is an OpenAccess journal.
Lipsitch, M., Lajous, M., O’Hagan, J., Cohen, T., Miller, J., Goldstein, E., Danon, L., Wallinga, J., Riley, S., Dowell, S., Reed, C., & McCarron, M. (2009). Use of Cumulative Incidence of Novel Influenza A/H1N1 in Foreign Travelers to Estimate Lower Bounds on Cumulative Incidence in Mexico PLoS ONE, 4 (9) DOI: 10.1371/journal.pone.0006895