Early outbreak of 2009 influenza A (H1N1) in Mexico prior to identification of pH1N1 virus

In the aftermath of the global spread of 2009 influenza A (pH1N1) virus, still very little is known of the early stages of the outbreak in Mexico during the early months of the year, before the virus was identified. We fit a simple mathematical model, the Richards model, to the number of excess labo...

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Bibliographic Details
Published in:PloS one 2011-08, Vol.6 (8), p.e23853-e23853
Main Authors: Hsieh, Ying-Hen, Ma, Stefan, Velasco Hernandez, Jorge X, Lee, Vernon J, Lim, Wei Yen
Format: Article
Language:English
Subjects:
Analysis
Biology
Continents
Criminal investigations
Disease control
Disease Outbreaks - statistics & numerical data
Epidemics
Epidemiology
Health aspects
Health surveillance
Humans
Incidence
Influenza
Influenza A
Influenza A Virus, H1N1 Subtype - physiology
Influenza, Human - epidemiology
Influenza, Human - virology
Mathematical models
Mathematics
Medical laboratories
Medicine
Mexico - epidemiology
Models, Biological
Public health
Reproduction
Respiratory diseases
Seasons
Swine flu
Time Factors
Viruses
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Summary:In the aftermath of the global spread of 2009 influenza A (pH1N1) virus, still very little is known of the early stages of the outbreak in Mexico during the early months of the year, before the virus was identified. We fit a simple mathematical model, the Richards model, to the number of excess laboratory-confirmed influenza cases in Mexico and Mexico City during the first 15 weeks in 2009 over the average influenza case number of the previous five baseline years of 2004-2008 during the same period to ascertain the turning point (or the peak incidence) of a wave of early influenza infections, and to estimate the transmissibility of the virus during these early months in terms of its basic reproduction number. The results indicate that there may have been an early epidemic in Mexico City as well as in all of Mexico during February/March. Based on excess influenza cases, the estimated basic reproduction number R₀ for the early outbreak was 1.59 (0.55 to 2.62) for Mexico City during weeks 5-9, and 1.25 (0.76, 1.74) for all of Mexico during weeks 5-14. We established the existence of an early epidemic in Mexico City and in all of Mexico during February/March utilizing the routine influenza surveillance data, although the location of seeding is unknown. Moreover, estimates of R₀ as well as the time of peak incidence (the turning point) for Mexico City and all of Mexico indicate that the early epidemic in Mexico City in February/March had been more transmissible (larger R₀) and peaked earlier than the rest of the country. Our conclusion lends support to the possibility that the virus could have already spread to other continents prior to the identification of the virus and the reporting of lab-confirmed pH1N1 cases in North America in April.
ISSN:1932-6203
1932-6203
DOI:10.1371/journal.pone.0023853