Epidemiological Characterization of a Fourth Wave of Pandemic A/H1N1 Influenza in Mexico, Winter 2011-2012: Age Shift and Severity

Víctor H. Borja-Aburto, Gerardo Chowell, Cécile Viboud, Lone Simonsen, Mark A. Miller, Concepción Grajales-Muñiz, Cesar R. González-Bonilla, Jose A. Diaz-Quiñonez, Santiago Echevarría-Zuno

Research output: Contribution to journalArticlepeer-review

30 Scopus citations


Background and Aims: A substantial recrudescent wave of pandemic influenza A/H1N1 affected the Mexican population from December 1, 2011-March 20, 2012 following a 2-year period of sporadic transmission. Methods: We analyzed demographic and geographic data on all hospitalizations with severe acute respiratory infection (SARI) and laboratory-confirmed A/H1N1 influenza, and inpatient deaths, from a large prospective surveillance system maintained by a Mexican social security medical system during April 1, 2009-March 20, 2012. We also estimated the reproduction number (R) based on the growth rate of the daily case incidence by date of symptoms onset. Results: A total of 7569 SARI hospitalizations and 443 in-patient deaths (5.9%) were reported between December 1, 2011, and March 20, 2012 (1115 A/H1N1-positive inpatients and 154 A/H1N1-positive deaths). The proportion of laboratory-confirmed A/H1N1 hospitalizations and deaths was higher among subjects ≥60 years of age (χ2 test, p <0.0001) and lower among younger age groups (χ2 test, p <0.04) for the 2011-2012 pandemic wave compared to the earlier waves in 2009. The reproduction number of the winter 2011-2012 wave in central Mexico was estimated at 1.2-1.3, similar to that reported for the fall 2009 wave, but lower than that of spring 2009. Conclusions: We documented a substantial increase in the number of SARI hospitalizations during the period December 2011-March 2012 and an older age distribution of laboratory-confirmed A/H1N1 influenza hospitalizations and deaths relative to 2009 A/H1N1 pandemic patterns. The gradual change in the age distribution of A/H1N1 infections in the post-pandemic period is consistent with a build-up of immunity among younger populations.

Original languageEnglish (US)
Pages (from-to)563-570
Number of pages8
JournalArchives of Medical Research
Issue number7
StatePublished - Oct 2012


  • A/H1N1 influenza pandemic
  • Age distribution
  • Deaths
  • Hospitalizations
  • Transmissibility

ASJC Scopus subject areas

  • Medicine(all)


Dive into the research topics of 'Epidemiological Characterization of a Fourth Wave of Pandemic A/H1N1 Influenza in Mexico, Winter 2011-2012: Age Shift and Severity'. Together they form a unique fingerprint.

Cite this