Use of cumulative incidence of novel influenza A/H1N1 in foreign travelers to estimate lower bounds on cumulative incidence in Mexico.
BACKGROUND
An accurate estimate of the total number of cases and severity of illness of an emerging infectious disease is required both to define the burden of the epidemic and to determine the severity of disease. When a novel pathogen first appears, affected individuals with severe symptoms are more likely to be diagnosed. Accordingly, the total number of cases will be underestimated and disease severity overestimated. This problem is manifest in the current epidemic of novel influenza A/H1N1.


RESULTS
We used a simple approach to leverage measures of incident influenza A/H1N1 among a relatively small and well observed group of US, UK, Spanish and Canadian travelers who had visited Mexico to estimate the incidence among a much larger and less well surveyed population of Mexican residents. We estimate that a minimum of 113,000 to 375,000 cases of novel influenza A/H1N1 have occurred in Mexicans during the month of April, 2009. Such an estimate serves as a lower bound because it does not account for underreporting of cases in travelers or for nonrandom mixing between Mexican residents and visitors, which together could increase the estimates by more than an order of magnitude.


CONCLUSIONS
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.
Lipsitch
Marc
M
Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts, United States of America.
Lajous
Martin
M
O'Hagan
Justin J
JJ
Cohen
Ted
T
Miller
Joel C
JC
Goldstein
Edward
E
Danon
Leon
L
Wallinga
Jacco
J
Riley
Steven
S
Dowell
Scott F
SF
Reed
Carrie
C
McCarron
Meg
M
D002170:Canada; D004196:Disease Outbreaks; D014943:Global Health; D006801:Humans; D015994:Incidence; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics*; D007251:Influenza, Human / Q000453:epidemiology* / Q000821:virology*; D007902:Length of Stay; D008800:Mexico; D011159:Population Surveillance; D013030:Spain; D014195:Travel; D006113:United Kingdom; D014481:United States