ABSTRACT
BACKGROUND: Recent studies have demonstrated the possibility of negative associations between prior influenza vaccines and subsequent influenza vaccine effectiveness (VE), depending on season and strain. We investigated this association over 4 consecutive influenza seasons (2011-2012 through 2014-2015) in Canada. METHODS: Using a matched test-negative design, laboratory-confirmed influenza cases and matched test-negative controls admitted to hospitals were enrolled. Patients were stratified into 4 groups according to influenza vaccine history (not vaccinated current and prior season [referent], vaccinated prior season only, vaccinated current season only, and vaccinated both current and prior season). Conditional logistic regression was used to estimate VE; prior vaccine impact was assessed each season for overall effect and effect stratified by age (<65 years, ≥65 years) and type/subtype (A/H1N1, A/H3N2, influenza B). RESULTS: Overall, mainly nonsignificant associations were observed. Trends of nonsignificant decreased VE among patients repeatedly vaccinated in both prior and current season relative to the current season only were observed in the A/H3N2-dominant seasons of 2012-2013 and 2014-2015. Conversely, in 2011-2012, during which B viruses circulated, and in 2013-2014, when A/H1N1 circulated, being vaccinated in both seasons tended to result in a high VE in the current season against the dominant circulating subtype. CONCLUSIONS: Prior vaccine impact on subsequent VE among Canadian inpatients was mainly nonsignificant. Even in circumstances where we observed a trend of negative impact, being repeatedly vaccinated was still more effective than not receiving the current season's vaccine. These findings favor continuation of annual influenza vaccination recommendations, particularly in older adults. CLINICAL TRIALS REGISTRATION: NCT01517191.
Subject(s)
Hospitalization , Influenza Vaccines/immunology , Influenza, Human/epidemiology , Influenza, Human/prevention & control , Seasons , Vaccination , Aged , Aged, 80 and over , Canada/epidemiology , Case-Control Studies , Female , Humans , Influenza A Virus, H1N1 Subtype/immunology , Influenza A Virus, H3N2 Subtype/immunology , Influenza B virus/immunology , Influenza Vaccines/administration & dosage , Influenza, Human/virology , Male , Middle Aged , Outcome Assessment, Health Care , Public Health Surveillance , Risk FactorsABSTRACT
Influenza is rarely laboratory-confirmed and the outpatient influenza burden is rarely studied due to a lack of suitable data. We used the Clinical Practice Research Datalink (CPRD) and surveillance data from Public Health England in a linear regression model to assess the number of persons consulting UK general practitioners (GP episodes) for respiratory illness, otitis media and antibiotic prescriptions attributable to influenza during 14 seasons, 1995-2009. In CPRD we ascertained influenza vaccination status in each season and risk status (conditions associated with severe influenza outcomes). Seasonal mean estimates of influenza-attributable GP episodes in the UK were 857 996 for respiratory disease including 68 777 for otitis media, with wide inter-seasonal variability. In an average season, 2·4%/0·5% of children aged <5 years and 1·3%/0·1% of seniors aged ⩾75 years had a GP episode for respiratory illness attributed to influenza A/B. Two-thirds of influenza-attributable GP episodes were estimated to result in prescription of antibiotics. These estimates are substantially greater than those derived from clinically reported influenza-like illness in surveillance programmes. Because health service costs of influenza are largely borne in general practice, these are important findings for cost-benefit assessment of influenza vaccination programmes.
Subject(s)
General Practice/statistics & numerical data , Influenza A virus , Influenza B virus , Influenza, Human/epidemiology , Otitis Media/epidemiology , Primary Health Care/statistics & numerical data , Adolescent , Adult , Age Factors , Aged , Anti-Bacterial Agents , Child , Child, Preschool , Comorbidity , Databases, Factual , Drug Prescriptions/statistics & numerical data , Humans , Incidence , Infant , Infant, Newborn , Influenza Vaccines , Influenza, Human/prevention & control , Influenza, Human/virology , Middle Aged , Otitis Media/drug therapy , Otitis Media/virology , Seasons , United Kingdom/epidemiology , Vaccination/statistics & numerical data , Young AdultSubject(s)
Hospitalization/statistics & numerical data , Influenza A Virus, H3N2 Subtype/isolation & purification , Influenza Vaccines/administration & dosage , Influenza, Human/prevention & control , Outcome Assessment, Health Care , Adolescent , Adult , Age Distribution , Aged , Aged, 80 and over , Canada/epidemiology , Case-Control Studies , Female , Humans , Influenza A Virus, H3N2 Subtype/immunology , Influenza Vaccines/immunology , Influenza, Human/epidemiology , Male , Middle Aged , Population Surveillance , Seasons , Sentinel Surveillance , Vaccination , Young AdultABSTRACT
During the 2013/14 influenza season in Canada, 631 of 654 hospitalisations for laboratory-confirmed influenza enrolled in sentinel hospitals were due to Influenza A. Of the 375 with known subtype, influenza A(H1N1) accounted for 357. Interim unmatched vaccine effectiveness adjusted for age and presence of one or more medical comorbidities was determined by test-negative case-control design to be 58.5% (90% confidence interval (CI): 43.9-69.3%) overall and 57.9% (90% CI: 37.7-71.5) for confirmed influenza A(H1N1).
