title: Household transmission of influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons.
abstract: BACKGROUND
  The transmission of influenza viruses occurs person to person and is facilitated by contacts within enclosed environments such as households. The aim of this study was to evaluate secondary attack rates and factors associated with household transmission of laboratory-confirmed influenza A(H1N1)pdm09 in the pandemic and post-pandemic seasons.
  
  
  METHODS
  During the 2009-2010 and 2010-2011 influenza seasons, 76 sentinel physicians in Navarra, Spain, took nasopharyngeal and pharyngeal swabs from patients diagnosed with influenza-like illness. A trained nurse telephoned households of those patients who were laboratory-confirmed for influenza A(H1N1)pdm09 to ask about the symptoms, risk factors and vaccination status of each household member.
  
  
  RESULTS
  In the 405 households with a patient laboratory-confirmed for influenza A(H1N1)pdm09, 977 susceptible contacts were identified; 16% of them (95% CI 14-19%) presented influenza-like illness and were considered as secondary cases. The secondary attack rate was 14% in 2009-2010 and 19% in the 2010-2011 season (p=0.049), an increase that mainly affected persons with major chronic conditions. In the multivariate logistic regression analysis, the risk of being a secondary case was higher in the 2010-2011 season than in the 2009-2010 season (adjusted odds ratio: 1.72; 95% CI 1.17-2.54), and in children under 5 years, with a decreasing risk in older contacts. Influenza vaccination was associated with lesser incidence of influenza-like illness near to statistical significance (adjusted odds ratio: 0.29; 95% CI 0.08-1.03).
  
  
  CONCLUSIONS
  The secondary attack rate in households was higher in the second season than in the first pandemic season. Children had a greater risk of infection. Preventive measures should be maintained in the second pandemic season, especially in high-risk persons.
authors.affiliation: Complejo Hospitalario de Navarra, Pamplona, Spain; Instituto de Salud Pública de Navarra, Pamplona, Spain.
authors.forename: Itziar
authors.initials: I
authors.lastname: Casado
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Madrid, Spain.
authors.forename: Iván
authors.initials: I
authors.lastname: Martínez-Baz
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Madrid, Spain.
authors.forename: Rosana
authors.initials: R
authors.lastname: Burgui
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Madrid, Spain.
authors.forename: Fátima
authors.initials: F
authors.lastname: Irisarri
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Madrid, Spain.
authors.forename: Maite
authors.initials: M
authors.lastname: Arriazu
authors.affiliation: Dirección de Atención Primaria, Servicio Navarro de Salud, Pamplona, Spain.
authors.forename: Fernando
authors.initials: F
authors.lastname: Elía
authors.affiliation: Complejo Hospitalario de Navarra, Pamplona, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Navascués
authors.affiliation: Complejo Hospitalario de Navarra, Pamplona, Spain.
authors.forename: Carmen
authors.initials: C
authors.lastname: Ezpeleta
authors.affiliation: Centro de Salud de San Juan, Servicio Navarro de Salud, Pamplona, Spain.
authors.forename: Pablo
authors.initials: P
authors.lastname: Aldaz
authors.affiliation: Instituto de Salud Pública de Navarra, Pamplona, Spain; CIBER Epidemiología y Salud Pública (CIBERESP), Madrid, Spain.
authors.forename: Jesús
authors.initials: J
authors.lastname: Castilla
mesh_terms: D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D005191:Family Characteristics; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission*; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance; D012307:Risk Factors; D012621:Seasons; D013030:Spain / Q000453:epidemiology; D055815:Young Adult