title: An estimate of the incidence of influenza-like illness during the influenza pandemic of 2009.
abstract: BACKGROUND
  The influenza pandemic of 2009 had a great social impact. Many health resources were devoted to the care, prevention and surveillance of this disease. Epidemiological surveillance is based on the reporting of cases of influenza-like illness (ILI) and confirmed influenza cases. The objective was to estimate the true incidence of ILI during the influenza pandemic of 2009.
  
  
  METHODS
  The capture-recapture method was applied during the month of highest influenza incidence in Castellón. Two notification systems were used: (i)electronic reporting of Notifiable Diseases (ND), and (ii)laboratory-based (LAB) data collection. Estimates were made by stratifying by age group and week. Independence coefficients were calculated for those strata.
  
  
  RESULTS
  No dependence was found between stratification variables and the reporting system. A total of 7,181 ND cases and 524 LAB cases were identified, of which 211 were recorded in both systems. The estimated total of cases was 17,785 in a single month. In the study period, almost 4% of people in the area suffered flu symptoms (cumulative incidence), with 1% being affected each day (daily prevalence). The sensitivity of the ND system was 40%, i.e., the percentage of patients seeking primary care.
  
  
  CONCLUSIONS
  To obtain an estimate of the actual incidence of influenza-like illness in the population during a pandemic period, the number of medical consultations should be multiplied by a factor of 2.5. This factor is lower than that estimated for periods without pandemic alert.
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España; Facultad de Medicina, Universidad Cardenal Herrera - CEU, Castellón, España; CIBER de Epidemiología y Salud Pública, CIBERESP, España. Electronic address: bellido_jua@gva.es.
authors.forename: Juan B
authors.initials: JB
authors.lastname: Bellido-Blasco
authors.affiliation: Servicio de Microbiología, Hospital General de Castellón, Castellón, España.
authors.forename: Francisco
authors.initials: F
authors.lastname: Pardo-Serrano
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España; CIBER de Epidemiología y Salud Pública, CIBERESP, España.
authors.forename: Isabel
authors.initials: I
authors.lastname: Ballester-Rodríguez
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España; CIBER de Epidemiología y Salud Pública, CIBERESP, España.
authors.forename: Alberto
authors.initials: A
authors.lastname: Arnedo-Pena
authors.affiliation: Servicio de Microbiología, Hospital General de Castellón, Castellón, España.
authors.forename: M Dolores
authors.initials: MD
authors.lastname: Tirado-Balaguer
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España.
authors.forename: M Ángeles
authors.initials: MÁ
authors.lastname: Romeu-García
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España.
authors.forename: Ester
authors.initials: E
authors.lastname: Silvestre-Silvestre
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España.
authors.forename: Noemí
authors.initials: N
authors.lastname: Meseguer-Ferrer
authors.affiliation: Sección de Epidemiología, Centro Salud Pública de Castellón, Castellón, España.
authors.forename: Concha
authors.initials: C
authors.lastname: Herrero-Carot
authors.affiliation: Servicio de Epidemiología, Agència de Salut Pública de Barcelona - ASPB, Barcelona, España; CIBER de Epidemiología y Salud Pública, CIBERESP, España.
authors.forename: Joan A
authors.initials: JA
authors.lastname: Caylà-Buqueres
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D019411:Clinical Laboratory Techniques / Q000706:statistics & numerical data; D003625:Data Collection; D016208:Databases, Factual; D018563:Disease Notification; D057286:Electronic Health Records; D005260:Female; D006801:Humans; D015994:Incidence; D007223:Infant; D007231:Infant, Newborn; D007251:Influenza, Human / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D058873:Pandemics; D010342:Patient Acceptance of Health Care / Q000706:statistics & numerical data; D011159:Population Surveillance / Q000379:methods*; D015995:Prevalence; D011320:Primary Health Care; D012141:Respiratory Tract Infections / Q000453:epidemiology; D013030:Spain / Q000453:epidemiology; D063189:Symptom Assessment; D055815:Young Adult
keywords: Captura-recaptura; Capture-recapture; Estimación; Estimation; Gripe; Incidence; Incidencia; Influenza; Influenza pandemic; Influenza-like syndrome; Pandemia de gripe; Síndrome gripal