title: [Transmissibility and severity of the pandemic influenza A (H1N1) 2009 virus in Spain].
abstract: OBJECTIVE
  To estimate the value of the basic reproduction number for the pandemic wave of influenza A (H1N1) 2009 in Spain and to assess its impact on morbidity and mortality in the Spanish population compared with those in the previous influenza season.
  
  
  METHODS
  Data on the incidence of influenza and viral detections were obtained from the Spanish Influenza Surveillance System. Deaths from pandemic influenza were obtained from the Coordinating Center for Health Alerts and Emergencies of the Spanish Ministry of Health and Social Policy, and deaths from seasonal influenza during the period 2003-2008 were obtained from the National Statistics Institute. The reproduction number was estimated by two methods: firstly, by using the growth rate of the cumulative incidence of influenza during the exponential growth phase of the pandemic wave, and secondly (maximum likelihood estimation), through analysis the dates of onset of symptoms observed in pairs of cases based on generation time distribution. We calculated the fatality rate and mortality from influenza by comparing potential years of life lost in the pandemic season with those in previous interpandemic seasons.
  
  
  RESULTS
  The start of the pandemic wave occurred in Spain earlier in week 40/2009 (from 4 to 10 October), with an absolute predominance of the new strain in the pattern of circulating viruses. The value of R(0) in the growth phase of the wave was 1.29 (95% CI: 1.25-1.33), estimated with the first method, and was 1.01 (95% CI: 0.99-1.03) with the second method. During the pandemic season, there were 318 deaths from pandemic influenza, affecting younger age groups than in previous interpandemic seasons. Consequently, the number of potential years of life lost in the pandemic season (11,612) was estimated at six times the adjusted annual average of the interpandemic influenza seasons for comparison (1,802).
  
  
  CONCLUSIONS
  The estimates of R(0) for the growth phase of the pandemic wave were in the lower range of estimates of this parameter in previous pandemics. Mortality indicators calculated in the pandemic period showed an increase in deaths compared with previous interpandemic seasons, which was most pronounced in young patients.
authors.affiliation: Área de Vigilancia de la Salud Pública, Centro Nacional de Epidemiología, Instituto de Salud Carlos III, Madrid, España. lorenas@isciii.es
authors.forename: Lorena
authors.initials: L
authors.lastname: Simón Méndez
authors.forename: Salvador
authors.initials: S
authors.lastname: de Mateo Ontañón
authors.forename: Amparo
authors.initials: A
authors.lastname: Larrauri Cámara
authors.forename: Silvia
authors.initials: S
authors.lastname: Jiménez-Jorge
authors.forename: Josep
authors.initials: J
authors.lastname: Vaqué Rafart
authors.forename: Santiago
authors.initials: S
authors.lastname: Pérez Hoyos
mesh_terms: D000293:Adolescent; D000328:Adult; D017677:Age Distribution; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000302:isolation & purification; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission / Q000821:virology; D008297:Male; D008875:Middle Aged; D009026:Mortality / Q000639:trends; D058873:Pandemics; D012621:Seasons; D013030:Spain / Q000453:epidemiology; D055815:Young Adult