Influenza vaccination coverage and factors associated with severe laboratory-confirmed influenza-related illness in patients receiving care at a tertiary hospital in Catalonia (Spain) during the 2018-2019 epidemic season

Introduction Influenza vaccination rates in risk groups remain suboptimal. Evidence supporting a significant association between influenza vaccination and severe illness is limited. Methods We retrospectively analyzed the epidemiological characteristics of out- and inpatients with laboratory-confirmed influenza infection attended during the 2018-19 epidemic season. Influenza vaccination coverage by indication was analyzed. Logistic regression was used to compare the odds of vaccination between severe and non-severe influenza-positive patients. Severe cases were defined as presenting pneumonia, admission to critical care units and/or death. Results The overall vaccination coverage among influenza-positive patients was 30.4%. In subjects with >= 1 indication for vaccination, the vaccination coverage was 42.4%. By indication, coverage rates were: 52.5% in patients aged >= 59 years, 42.2% in obese patients, 29.2% in immunosuppressed subjects and 6.5% in pregnant women. In patients with underlying chronic diseases, a higher coverage was found in patients with cognitive impairment (77%), muscular dystrophy (63.6%) and renal disease (60.4%). The multivariate logistic regression model showed severe influenza-related illness was associated with a lack of influenza vaccination before seeking care during the 2018-2019 season [0.59 (95%CI 0.36-0.97); p = 0.038], older age [1.01 (95%CI 1.00-1.02); p = 0.009] and current or former smoking status [1.63 (95%CI 0.84-3.18) and 2.03 (95%CI 1.16-3.57); p = 0.031], adjusted by underlying disease. Conclusion Adjusting by age, smoking status and underlying disease, a moderate association between the influenza vaccine and severe laboratory-confirmed influenza-related illness was found in an epidemic season in which there was matching between the vaccine and circulating strains. Protection against complications, especially in older subjects and in those with underlying disease is postulated as one of the strengths of annual influenza vaccination. However, influenza vaccination is a pending issue in these groups, especially in pregnant women and obese people. To avoid suboptimal vaccination coverages, health professionals should recommend the seasonal influenza vaccination according to the annual instructions of the health authorities.