title: Influenza vaccine effectiveness against laboratory confirmed influenza in Greece during the 2013-2014 season: a test-negative study.
abstract: BACKGROUND
  In 2013-2014 Greece experienced a resurgence of severe influenza cases, coincidental with a shift to H1N1pdm09 predominance. We sought to estimate Vaccine Effectiveness (VE) for this season using available surveillance data from hospitals (including both inpatients and outpatients).
  
  
  METHODS
  Swab samples were sent by hospital physicians to one of three laboratories, covering the entire country, to be tested for influenza using RT-PCR. The test-negative design was employed, with patients testing positive serving as cases and those testing negative serving as controls. VE was estimated using logistic regression, adjusted for age group, sex, region and calendar time, with further adjustment for unknown vaccination status using inverse response propensity weights. Additional age group stratified estimates and subgroup estimates of VE against H1N1pdm09 and H3N2 were calculated.
  
  
  RESULTS
  Out of 1310 patients with known vaccination status, 124 (9.5%) were vaccinated, and 543 patients (41.5%) tested positive for influenza. Adjusted VE was 34.5% (95% CI: 4.1-55.3%) against any influenza, and 56.7% (95% CI: 22.8-75.7%) against H1N1pdm09. VE estimates appeared to be higher for people aged 60 and older, while in those under 60 there was limited evidence of effectiveness. Isolated circulating strains were genetically close to the vaccine strain, with limited evidence of antigenic drift.
  
  
  CONCLUSIONS
  These results suggest a moderate protective effect of the 2013-2014 influenza vaccine, mainly against H1N1pdm09 and in people aged 60 and over. Vaccine coverage was very low in Greece, even among groups targeted for vaccination, and substantial efforts should be made to improve it. VE can and should be routinely monitored, and the results taken into account when deciding on influenza vaccine composition for next season.
authors.affiliation: Department of Epidemiological Surveillance and Intervention, Hellenic Centre for Disease Control and Prevention, Athens, Greece; Centre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain; Department of Experimental and Health Sciences, Universitat Pompeu Fabra (UPF), Barcelona, Spain. Electronic address: thlytras@gmail.com.
authors.forename: Theodore
authors.initials: T
authors.lastname: Lytras
authors.affiliation: National Influenza Reference Laboratory for Southern Greece, Hellenic Pasteur Institute, Athens, Greece.
authors.forename: Athanasios
authors.initials: A
authors.lastname: Kossyvakis
authors.affiliation: National Influenza Reference Laboratory for Northern Greece, Medical School, Aristotle University of Thessaloniki, Greece.
authors.forename: Angeliki
authors.initials: A
authors.lastname: Melidou
authors.affiliation: National Influenza Reference Laboratory for Northern Greece, Medical School, Aristotle University of Thessaloniki, Greece.
authors.forename: Maria
authors.initials: M
authors.lastname: Exindari
authors.affiliation: National Influenza Reference Laboratory for Northern Greece, Medical School, Aristotle University of Thessaloniki, Greece.
authors.forename: Georgia
authors.initials: G
authors.lastname: Gioula
authors.affiliation: National Influenza Reference Laboratory for Northern Greece, Medical School, Aristotle University of Thessaloniki, Greece.
authors.forename: Vasiliki
authors.initials: V
authors.lastname: Pogka
authors.affiliation: National Influenza Reference Laboratory for Northern Greece, Medical School, Aristotle University of Thessaloniki, Greece.
authors.forename: Nikolaos
authors.initials: N
authors.lastname: Malisiovas
authors.affiliation: National Influenza Reference Laboratory for Southern Greece, Hellenic Pasteur Institute, Athens, Greece.
authors.forename: Andreas
authors.initials: A
authors.lastname: Mentis
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D016022:Case-Control Studies; D002648:Child; D002675:Child, Preschool; D005260:Female; D006115:Greece / Q000453:epidemiology; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology; D053122:Influenza A Virus, H3N2 Subtype / Q000276:immunology; D007252:Influenza Vaccines / Q000276:immunology*; D007251:Influenza, Human / Q000517:prevention & control*; D016015:Logistic Models; D008297:Male; D008875:Middle Aged; D010802:Phylogeny; D011159:Population Surveillance; D012621:Seasons; D018571:Sentinel Surveillance; D013997:Time Factors; D014611:Vaccination; D055815:Young Adult
chemical_list: D007252:Influenza Vaccines
keywords: Epidemiology; Influenza; Influenza-like illness—ILI; Molecular methods; Surveillance; Vaccine effectiveness