title: Intense seasonal A/H1N1 influenza in Mexico, winter 2013-2014.
abstract: OBJECTIVE
  A recrudescent wave of pandemic influenza A/H1N1 affected Mexico during the winter of 2013-2014 following a mild 2012-2013 A/H3N2 influenza season.
  
  
  METHODS
  We compared the demographic and geographic characteristics of hospitalizations and inpatient deaths for severe acute respiratory infection (SARI) and laboratory-confirmed influenza during the 2013-2014 influenza season compared to previous influenza seasons, based on a large prospective surveillance system maintained by the Mexican Social Security health care system.
  
  
  RESULTS
  A total of 14,236 SARI hospitalizations and 1,163 inpatient deaths (8.2%) were reported between October 1, 2013 and March 31, 2014. Rates of laboratory-confirmed A/H1N1 hospitalizations and deaths were significantly higher among individuals aged 30-59 years and lower among younger age groups for the 2013-2014 A/H1N1 season compared to the previous A/H1N1 season in 2011-2012 (χ(2) test, p <0.001). The reproduction number for the winter 2013-2014 influenza season in central Mexico was estimated at 1.3-1.4, in line with that reported for the 2011-2012 A/H1N1 season but lower than during the initial waves of pandemic A/H1N1 activity in 2009.
  
  
  CONCLUSIONS
  We documented a substantial increase in the number of A/H1N1-related hospitalizations and deaths during the period from October 2013-March 2014 in Mexico and a proportionate shift of severe disease to middle-aged adults, relative to the preceding A/H1N1 2011-2012 season. In the absence of clear antigenic drift in globally circulating A/H1N1 viruses in the post-2009 pandemic period, the gradual change in the age distribution of A/H1N1 infections observed in Mexico suggests a slow build-up of immunity among younger populations, reminiscent of the age profile of past pandemics.
authors.affiliation: Direction of Medical Benefits, Mexican Institute of Social Security, Mexico City, Mexico.
authors.forename: Javier
authors.initials: J
authors.lastname: Dávila-Torres
authors.affiliation: Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA; Mathematical, Computational and Modeling Sciences Center, School of Human Evolution and Social Change, Arizona State University, Tempe, Arizona, USA. Electronic address: gchowell@asu.edu.
authors.forename: Gerardo
authors.initials: G
authors.lastname: Chowell
authors.affiliation: Direction of Medical Benefits, Mexican Institute of Social Security, Mexico City, Mexico.
authors.forename: Víctor H
authors.initials: VH
authors.lastname: Borja-Aburto
authors.affiliation: Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA.
authors.forename: Cécile
authors.initials: C
authors.lastname: Viboud
authors.affiliation: Coordination of Epidemiological Surveillance and Contingency Support, Mexican Institute of Social Security, Mexico, City, Mexico.
authors.forename: Concepción
authors.initials: C
authors.lastname: Grajalez-Muñiz
authors.affiliation: Division of Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, Bethesda, Maryland, USA.
authors.forename: Mark A
authors.initials: MA
authors.lastname: Miller
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D005843:Geography; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000821:virology; D007297:Inpatients; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D012141:Respiratory Tract Infections / Q000473:pathology / Q000821:virology; D055815:Young Adult
keywords: A/H1N1 influenza pandemic; Age distribution; Drift; Immunity; Influenza vaccine; Transmissibility