title: Infection and death from influenza A H1N1 virus in Mexico: a retrospective analysis.
abstract: BACKGROUND
  In April, 2009, the first cases of influenza A H1N1 were registered in Mexico and associated with an unexpected number of deaths. We report the timing and spread of H1N1 in cases, and explore protective and risk factors for infection, severe disease, and death.
  
  
  METHODS
  We analysed information gathered by the influenza surveillance system from April 28 to July 31, 2009, for patients with influenza-like illness who attended clinics that were part of the Mexican Institute for Social Security network. We calculated odds ratios (ORs) to compare risks of testing positive for H1N1 in those with influenza-like illness at clinic visits, the risk of admission for laboratory-confirmed cases of H1N1, and of death for inpatients according to demographic characteristics, clinical symptoms, seasonal influenza vaccine status, and elapsed time from symptom onset to admission.
  
  
  RESULTS
  By July 31, 63 479 cases of influenza-like illness were reported; 6945 (11%) cases of H1N1 were confirmed, 6407 (92%) were outpatients, 475 (7%) were admitted and survived, and 63 (<1%) died. Those aged 10-39 years were most affected (3922 [56%]). Mortality rates showed a J-shaped curve, with greatest risk in those aged 70 years and older (10.3%). Risk of infection was lowered in those who had been vaccinated for seasonal influenza (OR 0.65 [95% CI 0.55-0.77]). Delayed admission (1.19 [1.11-1.28] per day) and presence of chronic diseases (6.1 [2.37-15.99]) were associated with increased risk of dying.
  
  
  CONCLUSIONS
  Risk communication and hospital preparedness are key factors to reduce mortality from H1N1 infection. Protective effects of seasonal influenza vaccination for the virus need to be investigated.
  
  
  BACKGROUND
  None.
authors.affiliation: Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Santiago
authors.initials: S
authors.lastname: Echevarría-Zuno
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Juan Manuel
authors.initials: JM
authors.lastname: Mejía-Aranguré
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Alvaro J
authors.initials: AJ
authors.lastname: Mar-Obeso
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Concepción
authors.initials: C
authors.lastname: Grajales-Muñiz
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Eduardo
authors.initials: E
authors.lastname: Robles-Pérez
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Margot
authors.initials: M
authors.lastname: González-León
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Manuel Carlos
authors.initials: MC
authors.lastname: Ortega-Alvarez
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Cesar
authors.initials: C
authors.lastname: Gonzalez-Bonilla
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico.
authors.forename: Ramón Alberto
authors.initials: RA
authors.lastname: Rascón-Pacheco
authors.affiliation: Coordinación de Vigilancia Epidemiológica y Apoyo en Contingencias, Unidad de Salud Pública, Dirección de Prestaciones Médicas, Instituto Mexicano del Seguro Social, Mexico City, Mexico. Electronic address: victor.borja@imss.gob.mx.
authors.forename: Víctor Hugo
authors.initials: VH
authors.lastname: Borja-Aburto
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D002908:Chronic Disease; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype; D007252:Influenza Vaccines; D007251:Influenza, Human / Q000453:epidemiology* / Q000401:mortality / Q000517:prevention & control; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D012189:Retrospective Studies
chemical_list: D007252:Influenza Vaccines