title: Use of cumulative incidence of novel influenza A/H1N1 in foreign travelers to estimate lower bounds on cumulative incidence in Mexico.
abstract: 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.
authors.affiliation: Department of Immunology and Infectious Diseases, Harvard School of Public Health, Boston, Massachusetts, United States of America.
authors.forename: Marc
authors.initials: M
authors.lastname: Lipsitch
authors.forename: Martin
authors.initials: M
authors.lastname: Lajous
authors.forename: Justin J
authors.initials: JJ
authors.lastname: O'Hagan
authors.forename: Ted
authors.initials: T
authors.lastname: Cohen
authors.forename: Joel C
authors.initials: JC
authors.lastname: Miller
authors.forename: Edward
authors.initials: E
authors.lastname: Goldstein
authors.forename: Leon
authors.initials: L
authors.lastname: Danon
authors.forename: Jacco
authors.initials: J
authors.lastname: Wallinga
authors.forename: Steven
authors.initials: S
authors.lastname: Riley
authors.forename: Scott F
authors.initials: SF
authors.lastname: Dowell
authors.forename: Carrie
authors.initials: C
authors.lastname: Reed
authors.forename: Meg
authors.initials: M
authors.lastname: McCarron
mesh_terms: 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