title: Clinical and epidemiological features of 2009 pandemic H1N1 influenza differ slightly according to seroprevalence status during the second wave in the general population in México.
abstract: BACKGROUND
  Clinical features of pandemic H1N1 have been derived from lab-confirmed, hospitalized, or critically ill subjects. This report describes the clinical features of H1N1 and their prevalence from non-confirmed subjects according to seroprevalence status in México. The  objective was to determine the prevalence of these clinical features from non-confirmed cases of pandemic H1N1 and to compare them according to seroprevalence status in northern Monterrey, México, during 2009, and to identify the predictive signs and symptoms; there have been no prior  serologic studies in México.
  
  
  METHODS
  During November-December 2009, 2,222 volunteers, ages 6-99 years, were categorized into 3 symptomatic groups: influenza-like illness, respiratory illness, and non-respiratory illness. Antibodies against influenza A/H1N1/2009 were determined  by a virus-free enzyme-linked immunosorbent assay (ELISA) method. Demographics and clinical presentation were assessed through face-to-face questionnaire, and the association with seroprevalence status was determined and compared.
  
  
  RESULTS
  Overall seroprevalence was 39%. Of the seropositive  subjects, 67% were symptomatic and 33% were asymptomatic. Seventy-one percent of seropositive symptomatic subjects reported respiratory illness, 17% reported non-respiratory symptoms, and 12% reported influenza-like illness. The most common symptoms were rhinorrhea/nasal congestion (93%) and  headache (83%). No significant difference was found between the symptom profiles of the seropositive group, compared to the seronegative one, nor of the median duration of symptoms. The seropositive group had a significantly elevated proportion of influenza-like illness (12%), compared to  the seronegative group (8%). The proportion of subjects who took days off and who sought medical attention was significantly higher in the seropositive group. No single symptom was associated as a predictor of seropositiveness.
  
  
  CONCLUSIONS
  One third of the seropositive subjects were asymptomatic,  and few had an influenza-like illness. No difference was found in the symptom profiles of the seropositive and seronegative groups. No single symptom predicted seropositiveness. Large scale population studies are needed, especially in México, to characterize clinical syndromes.
authors.affiliation: Clinical Research Center, School of Medicine and Health Sciences, Tecnológico de Monterrey, Nuevo León, México. lelizond@itesm.mx
authors.forename: Leticia
authors.initials: L
authors.lastname: Elizondo-Montemayor
authors.forename: Martín
authors.initials: M
authors.lastname: Hernández-Torre
authors.forename: Patricia A
authors.initials: PA
authors.lastname: Ugalde-Casas
authors.forename: Jesús
authors.initials: J
authors.lastname: Santos-Guzmán
authors.forename: Mónica
authors.initials: M
authors.lastname: Serrano-González
authors.forename: Norma G
authors.initials: NG
authors.lastname: Gutiérrez
authors.forename: Lorena
authors.initials: L
authors.lastname: Lam-Franco
authors.forename: Daniela
authors.initials: D
authors.lastname: Tamargo-Barrera
authors.forename: Ubaldo
authors.initials: U
authors.lastname: Martínez
authors.forename: Humberto
authors.initials: H
authors.lastname: Bustamante-Careaga
authors.forename: Mario M
authors.initials: MM
authors.lastname: Alvarez
authors.forename: Stacey
authors.initials: S
authors.lastname: Schultz-Cherry
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D018771:Arthralgia / Q000453:epidemiology / Q000821:virology; D058345:Asymptomatic Infections / Q000453:epidemiology*; D002648:Child; D005260:Female; D006261:Headache / Q000453:epidemiology / Q000821:virology; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000453:epidemiology* / Q000503:physiopathology / Q000821:virology; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D059352:Musculoskeletal Pain / Q000453:epidemiology / Q000821:virology; D015508:Nasal Obstruction / Q000453:epidemiology / Q000821:virology; D010342:Patient Acceptance of Health Care / Q000706:statistics & numerical data; D012140:Respiratory Tract Diseases / Q000453:epidemiology / Q000821:virology; D016036:Seroepidemiologic Studies; D018582:Sick Leave / Q000706:statistics & numerical data; D011795:Surveys and Questionnaires; D055815:Young Adult