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Flu Vaccine Less Effective among Elderly
Morbidity from influenza is significant in people ³65 years of age. As a result, annual vaccination is recommended to reduce the burden of influenza in this patient population. New analysis suggests, however, that the influenza vaccination may be associated with reductions in the composite of pneumonia and influenza (P&I) hospitalization and death among the elderly, but not with all-cause mortality alone, according to study results reported online first in the Archives of Internal Medicine [doi:10.001/archintermed.2011.2038].
Past observational studies suggest that influenza vaccines reduce all-cause mortality in the elderly by approximately 50%. Recent studies indicated an association between influenza vaccinations and mortality reduction among the elderly even during noninfluenza seasons, suggesting potential bias in such studies. To obtain an unbiased estimate of influenza vaccine effectiveness against all-cause mortality and hospitalizations for P&I in elderly patients, researchers used a technique known as instrumental variable (IV) analysis, which is designed to eliminate bias by controlling for unmeasured confounding.
Researchers conducted a population-based cohort study of 9 influenza seasons from 2000 to 2001 and 2008 to 2009 using health administrative databases in Ontario to examine the association between influenza and all-cause mortality among 12.6 million individuals aged 65 and older. Exclusion criteria included institutionalized individuals, those with invalid health card numbers, and individuals with no contact with the healthcare system within 3 years before an index date. The investigators used logistic regression modeling and IV analysis to remove the effect of selection bias. The IV was census subdivision (CSD)-specific vaccine coverage, defined as the percentage of people aged ≥65 years living in a specific CSD in Ontario who received the vaccine in a given influenza season. The main outcome measure was all-cause mortality during influenza season.
Of the overall data gathered on 12,621,806 individuals, 58.2% had been vaccinated. There were 130,532 deaths and 62,913 P&I hospitalizations during influenza seasons. With standard regression modeling, vaccination was associated with a 33% reduction in mortality during influenza seasons (adjusted odds ratio [OR], 0.67; 95% confidence interval [CI], 0.62-0.72). Influenza vaccination was also significantly associated with mortality reduction after influenza seasons (adjusted OR, 0.85; 95% CI, 0.83-0.86).
On IV analysis, the vaccination was associated with a 6% reduction in mortality during the influenza seasons, which was not significant (adjusted OR, 0.94; 95% CI, 0.84-1.04). Furthermore, the IV analysis showed no correlation between the vaccination and any reduction in mortality after the influenza seasons (adjusted OR, 1.13; 95% CI, 1.07-1.19). Data were adjusted for demographics, comorbidities, previous use of healthcare services, prescription medication use, and receipt of specific medical procedures.
For the composite outcome of P&I hospitalization and death, regression modeling showed reductions associated with the vaccine during influenza seasons (adjusted OR, 0.74; 95% CI, 0.70-0.78) and during postinfluenza seasons (adjusted OR, 0.88; 95% CI, 0.87-0.90). IV analysis showed a reduction during influenza seasons (adjusted OR, 0.86; 95% CI, 0.79-0.92), but no reduction after the influenza seasons (adjusted OR, 1.02; 95% CI, 0.97-1.06).
Limitations cited by the authors include individual-level vaccination status, and regional vaccine coverage may have been misclassified to a certain extent because approximately 25% of influenza vaccines administered to the elderly are not recorded in the Ontario Health Insurance Plan physician claims database. Also cause-specific mortality data were unavailable; therefore, the researchers could not use P&I or respiratory mortality as more specific outcomes.
“Current guidelines recommending annual vaccination against influenza for elderly individuals should remain in place until more definitive evidence has been amassed, as even small reductions in outcomes during influenza season as a result of influenza vaccination would be worthwhile because it is a generally safe and relatively low-cost intervention,” concluded the investigators.
