Effect of Influenza Vaccination in Preventing Laboratory-Confirmed Influenza Hospitalization in Patients With Diabetes Mellitus

Background. People with diabetes are at high risk of severe influenza complications. The influenza vaccination effect among diabetic patients remains inconclusive. We estimated the average effect of influenza vaccination status in the current and prior seasons in preventing laboratory-confirmed influenza hospitalization in diabetic patients. Methods. Patients attended in hospitals and primary healthcare centers with influenza-like illness were tested for influenza from the 2013-2014 to 2018-2019 seasons in Navarre, Spain. A test-negative case-control design in diabetic inpatients compared the influenza vaccination status in the current and 5 prior seasons between laboratory-confirmed influenza cases and negative controls. Vaccination status of influenza-confirmed cases was compared between diabetic inpatients and outpatients. Influenza vaccination effect was compared between diabetic patients and older (>= 60 years) or chronic nondiabetic patients. Results. Of 1670 diabetic inpatients tested, 569 (34%) were confirmed for influenza and 1101 were test-negative controls. The average effect in preventing influenza hospitalization was 46% (95% confidence interval [CI], 28%-59%) for current-season vaccination and 44% (95% CI, 20%-61%) for vaccination in prior seasons only in comparison to unvaccinated patients in the current and prior seasons. Among diabetic patients with confirmed influenza, current-season vaccination reduced the probability of hospitalization (adjusted odds ratio, 0.35; 95% CI,.15-.79). In diabetic patients, vaccination effect against influenza hospitalizations was not inferior to that in older or chronic nondiabetic patients. Conclusions. On average, influenza vaccination of diabetic population reduced by around half the risk of influenza hospitalization. Vaccination in prior seasons maintained a notable protective effect. These results reinforce the recommendation of influenza vaccination for diabetic patients.