The associations of previous influenza/upper respiratory infection with COVID-19 susceptibility/morbidity/mortality: a nationwide cohort study in South Korea.

We aimed to investigate the associations of previous influenza/URI with the susceptibility of COVID-19 patients compared to that of non-COVID-19 participants. A nationwide COVID-19 cohort database was collected by the Korea National Health Insurance Corporation. A total of 8,070 COVID-19 patients (1 January 2020 through 4 June 2020) were matched with 32,280 control participants. Severe COVID-19 morbidity was defined based on the treatment histories of the intensive care unit, invasive ventilation, and extracorporeal membrane oxygenation and death. The susceptibility/morbidity/mortality associated with prior histories of 1-14, 1-30, 1-90, 15-45, 15-90, and 31-90 days before COVID-19 onset were analyzed using conditional/unconditional logistic regression. Prior influenza infection was related to increased susceptibility to COVID-19 (adjusted odds ratio [95% confidence interval] = 3.07 [1.61-5.85] for 1-14 days and 1.91 [1.54-2.37] for 1-90 days). Prior URI was also associated with increased susceptibility to COVID-19 (6.95 [6.38-7.58] for 1-14 days, 4.99 [4.64-5.37] for 1-30 days, and 2.70 [2.55-2.86] for 1-90 days). COVID-19 morbidity was positively associated with influenza (3.64 [1.55-9.21] and 3.59 [1.42-9.05]) and URI (1.40 [1.11-1.78] and 1.28 [1.02-1.61]) at 1-14 days and 1-30 days, respectively. Overall, previous influenza/URI did not show an association with COVID-19 mortality. Previous influenza/URI histories were associated with increased COVID-19 susceptibility and morbidity. Our findings indicate why controlling influenza/URI is important during the COVID-19 pandemic.