title: [Influence of distinct criteria for selecting patients for swabbing on estimation of the effectiveness of the influenza vaccine].
abstract: OBJECTIVE
  To estimate the effectiveness of the influenza vaccine under different criteria for selecting patients for swabbing.
  
  
  METHODS
  A case-control study was performed of laboratory-confirmed cases (n=909) and negative controls for influenza (n=732) in the 2010-2011 to 2012-2013 seasons in Navarre (Spain). The adjusted vaccine effectiveness was estimated by including all swabs from patients with influenza-like-illness and selecting only the first two cases per physician and week.
  
  
  RESULTS
  The first two patients per physician and week were less frequently vaccinated against influenza (7.9% vs. 12.5%, p=0.021) and less often received confirmation of influenza (53.6% vs. 66.4%, p <0.001) than subsequent patients. These differences decreased after adjustment for covariates. The effectiveness of the influenza vaccine was 49% (95% CI: 23-66%) when all swabs were included and was 55% (95% CI: 27-72%) when we selected the first two swabs per week and physician.
  
  
  CONCLUSIONS
  The selection of the first two patients per physician and week may bias assessment of the effectiveness of the influenza vaccine, although this bias was small in the seasons analyzed.
authors: Instituto de Salud Pública de Navarra, Pamplona, España; CIBER de Epidemiología y Salud Pública (CIBERESP), España. Electronic address: imartinba@navarra.es.
authors: Iván
authors: I
authors: Martínez-Baz
authors: Instituto de Salud Pública de Navarra, Pamplona, España; CIBER de Epidemiología y Salud Pública (CIBERESP), España.
authors: Marcela
authors: M
authors: Guevara
authors: Dirección de Atención Primaria, Servicio Navarro de Salud, Pamplona, España.
authors: Fernando
authors: F
authors: Elía
authors: Servicio de Microbiología Clínica, Complejo Hospitalario de Navarra, Pamplona, España.
authors: Carmen
authors: C
authors: Ezpeleta
authors: Servicio de Microbiología Clínica, sección de Virología, Clínica Universidad de Navarra, Pamplona, España.
authors: Mirian
authors: M
authors: Fernández Alonso
authors: Instituto de Salud Pública de Navarra, Pamplona, España; CIBER de Epidemiología y Salud Pública (CIBERESP), España.
authors: Jesús
authors: J
authors: Castilla
mesh_terms: D000293:Adolescent; D000328:Adult; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D007252:Influenza Vaccines / Q000276:immunology*; D007251:Influenza, Human / Q000517:prevention & control*; D008875:Middle Aged; D009305:Nasopharynx / Q000821:virology*; D009975:Orthomyxoviridae / Q000302:isolation & purification*; D018579:Patient Selection; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines
keywords: Case control study; Diagnóstico de laboratorio; Estudio de casos y controles; Estudios de muestreo; Gripe; Influenza; Influenza vaccine; Laboratory diagnoses; Sampling studies; Selection bias; Sentinel surveillance; Sesgo de selección; Vacuna antigripal; Vigilancia centinela