Influenza: Diabetes as a risk factor for severe related-outcomes and the effectiveness of vaccination in diabetic population. A meta-analysis of observational studies

Aims: In order to better define the need for influenza vaccination in people with diabetes (DM), we collected all available evidence on the effect of DM as a risk factor for complications of both seasonal and pandemic influenza, and on the specific effectiveness of vaccines in patients with DM. Data synthesis: Two distinct systematic searches on MEDLINE, Cochrane, ClinicalTrials.gov and Embase databases were performed, one for each metanalysis, collecting all observational studies and randomized clinical trials performed on humans up to May 31st, 2022. We retrieved 34 observational studies comparing risk for influenza complications in people with or without diabetes, and 13 observational studies assessing vaccine effectiveness on preventing such complications. Mortality for influenza and hospitalization for influenza and pneumonia resulted significantly higher in individuals with versus without DM, both when unadjusted and adjusted data are analyzed. In diabetic individuals vaccinated for influenza overall hospitalization, hospitalization for influenza or pneumonia and overall mortality are significantly lower in comparison with not vaccinated DM subjects, both when unadjusted and adjusted data were analyzed. Conclusion: This systematic review and meta-analysis shows that: 1) influenza is associated with more severe complications in diabetic versus not diabetic individuals and 2) influenza vaccination is effective in preventing clinically relevant outcomes in adults with DM with a NNT (number needed to treat) of 60, 319, and 250 for all-cause hospitalization, specific hospitalization, and all-cause mortality, respectively. The identification of diabetic patients as the target of vaccination campaigns for influenza appears to be justified by available clinical evidence.