Influenza vaccination and the risk of COVID-19 infection and severe illness in older adults in the United States.

The coronavirus disease of 2019 (COVID-19) has caused a global pandemic and led to nearly three million deaths globally. As of April 2021, there are still many countries that do not have COVID-19 vaccines. Before the COVID-19 vaccines were developed, some evidence suggested that an influenza vaccine may stimulate nonspecific immune responses that reduce the risk of COVID-19 infection or the severity of COVID-19 illness after infection. This study evaluated the association between influenza vaccination and the risk of COVID-19 infection. We conducted a retrospective cross-sectional study with data from July 1, 2019, to June 30, 2020 with the Claims data from Symphony Health database. The study population was adults age 65 years old or older who received influenza vaccination between September 1 and December 31 of 2019. The main outcomes and measures were odds of COVID-19 infection and severe COVID-19 illness after January 15, 2020. We found the adjusted odds ratio (aOR) of COVID-19 infection risk between the influenza-vaccination group and no-influenza-vaccination group was 0.76 (95% confidence interval (CI), 0.75-0.77). Among COVID-19 patients, the aOR of developing severe COVID-19 illness was 0.72 (95% CI, 0.68-0.76) between the influenza-vaccination group and the no-influenza-vaccination group. When the influenza-vaccination group and the other-vaccination group were compared, the aOR of COVID-19 infection was 0.95 (95% CI, 0.93-0.97), and the aOR of developing a severe COVID-19 illness was 0.95 (95% CI, 0.80-1.13). The influenza vaccine may marginally protect people from COVID-19 infection.