A high-dose influenza vaccine was more effective than standard-dose vaccine in protecting a Veterans Health Administration (VHA) senior population from influenza- or pneumonia-associated hospitalization, according to research published in the Journal of Infectious Diseases.
A retrospective cohort of VHA patients, ≥65 years old, vaccinated in the 2015 to 2016 influenza season was used to study relative vaccine effectiveness. Adjustment for potential confounders was done by matching each high-dose patient with up to 4 standard-dose patients, vaccinated at the same location, within 2 weeks and sharing 2 or more preexisting medical comorbidities.
Matching resulted in 49,091 standard-dose and 24,682 high-dose patients. Previous event rate ratio-adjusted relative vaccine effectiveness was 25% (95% CI, 2%-43%) and 14% (95% CI, −8% to 32%) against influenza- and pneumonia-associated hospitalization or outpatient visits, respectively. Results were 7% (95% CI, −2% to 14%) and 5% (95% CI, 2%-8%) against all-cause hospitalization or outpatient visits, respectively, and 38% (95% CI, −5% to 65%) against laboratory-confirmed influenza.
The use of the previous event rate ratio method helped to “adjust for confounding by indication induced by unmeasured variables,” but could not be applied to the very specific outcome of laboratory-confirmed influenza, as this was not observed at baseline. This may have resulted in 38% being an underestimation. Also, the generalizability of the study should be interpreted with caution, as the cohort was drawn only from VHA facilities. Nevertheless, of the 5 previously studied influenza seasons, high-dose vaccines were more effective in all but one.
Further research is still needed, as the severity and types of circulating strains vary year by year, and investigators plan to look at high-dose effectiveness within 3 months from the end of a season.
Young-Xu Y, Van Aalst R, Mahmud SM, et al. Relative vaccine effectiveness of high-dose versus standard-dose influenza vaccines among Veterans Health Administration patients [published online February 14, 2018]. J Infect Dis. doi:10.1093/infdis/jiy088