Abstract
BackgroundSome studies have evaluated vaccine effectiveness in preventing outpatient influenza while others have analysed its effectiveness in preventing hospitalizations. This study evaluates the effectiveness of the trivalent influenza vaccine in preventing outpatient illness and hospitalizations from laboratory-confirmed influenza in the 2010–2011 season.MethodsWe conducted a nested case–control study in the population covered by the general practitioner sentinel network for influenza surveillance in Navarre, Spain. Patients with influenza-like illness in hospitals and primary health care were swabbed for influenza testing. Influenza vaccination status and other covariates were obtained from health care databases. Using logistic regression, the vaccination status of laboratory-confirmed influenza cases was compared with that of test-negative controls, adjusting for age, sex, comorbidity, outpatient visits in the previous 12 months, health care setting, time between symptom onset and swabbing, period and A(H1N1)pdm09 vaccination. Effectiveness was calculated as (1-odds ratio)x100.ResultsThe 303 confirmed influenza cases (88% for A(H1N1)pdm09 influenza) were compared with the 286 influenza test-negative controls. The percentage of persons vaccinated against influenza was 4.3% and 15.7%, respectively (p<0.001). The adjusted estimate of effectiveness was 67% (95% CI: 24%, 86%) for all patients and 64% (95% CI: 8%, 86%) in those with an indication for vaccination (persons age 60 or older or with major chronic conditions). Having received both the 2010–2011 seasonal influenza vaccine and the 2009–2010 pandemic influenza vaccine provided 87% protection (95% CI: 30%, 98%) as compared to those not vaccinated.ConclusionThe 2010–2011 seasonal influenza vaccine had a moderate protective effect in preventing laboratory-confirmed influenza.
Highlights
Some studies have evaluated vaccine effectiveness in preventing outpatient influenza while others have analysed its effectiveness in preventing hospitalizations
Compared with confirmed cases of influenza, testnegative controls had a higher proportion of persons who were age 65 or over, who had consulted a physician five or more times in the last year, who had more than one major chronic condition, who were diagnosed between week 51 of 2010 and 2 of 2011, and who were treated in the hospital
A smaller proportion of cases than controls had been vaccinated against influenza A(H1N1)pdm2009, and this was true for the 2010–2011 seasonal vaccine
Summary
Some studies have evaluated vaccine effectiveness in preventing outpatient influenza while others have analysed its effectiveness in preventing hospitalizations. This study evaluates the effectiveness of the trivalent influenza vaccine in preventing outpatient illness and hospitalizations from laboratory-confirmed influenza in the 2010–2011 season. Annual vaccination is the primary measure for preventing influenza and its complications [3]. In the absence of studies have applied this methodology in patients recruited in primary care, which provides an estimate of the effect of the vaccine in preventing mild cases [9,10,11,12]. Other studies have analyzed hospitalized patients, which provides an estimate of vaccine effectiveness in preventing serious forms of illness [13,14]. This study uses a test-negative case–control design nested in a population-based cohort to evaluate the effectiveness of the influenza vaccine in the 2010–2011 season in preventing laboratory-confirmed influenza, including both outpatient and hospitalized patients
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