Preventive effects of influenza and pneumococcal vaccination in the elderly – results from a population-based retrospective cohort study

Influenza and pneumococcal vaccinations are recommended in the elderly to reduce life-threatening complications like sepsis. Protection may be reduced with increasing age. We aimed to assess the effectiveness of both vaccines in the elderly by performing a retrospective cohort study of 138,877 individuals aged ≥60 y in Germany, who were insured in a large statutory health insurance (AOK PLUS). We used longitudinal claims data to classify individuals according to vaccination status 2008–2014, and assessed vaccine effectiveness (VE) in 2015 and 2016. Inverse probability weighting based on generalized propensity scores was used to adjust for systematic between-group differences. Influenza vaccination was associated with a reduction of hospital treatment in laboratory-confirmed influenza in 2015 (VE = 41.32 [95%CI 0.85, 65.26]), but had no significant impact on the overall influenza incidence. Complications of influenza (pneumonia and sepsis) were reduced in 2016. We found a rise in influenza-like illness and acute respiratory infections in both years and an increased 90-d mortality after hospital-treated pneumonia in vaccinees in 2015. Pneumococcal vaccination was effective in preventing hospital-treated pneumonia within the first and second year after vaccination (VE = 52.45 [13.31, 73.92] and 46.04 [5.46, 69.21], respectively), but had no impact on sepsis incidence or pneumonia mortality. Influenza and pneumococcal vaccination can prevent severe complications from influenza and hospital-treated pneumonia in the elderly, respectively. Vaccine effects differ between years and seasons and are partly difficult to interpret. Despite extensive efforts to adjust for between-group differences, residual bias cannot be ruled out, possibly explaining signals like increased ILI or pneumonia mortality.