Influenza viral infection is a risk factor for severe illness in COVID-19 patients: a nationwide population-based cohort study

ABSTRACT

In order to prepare for the twindemic of influenza and SARS-CoV-2 infection, we investigated the association between influenza infection and subsequent severity of SARS-CoV-2 infection. A population-based nationwide cohort study was performed using data from the National Health Insurance Service (NHIS) in the Republic of Korea. This study included 274,126 individuals who underwent SARS-CoV-2 PCR testing between 20 January 2020 and 1 October 2020. Among these patients, 28,338 tested positive for SARS-CoV-2, and 4,003 of these individuals had a history of influenza. The control group was selected through 1:1 propensity score matching. In the group of 4,003 COVID-19-positive individuals with no history of influenza, 192 (4.8%) experienced severe illness from COVID-19 infection. In the group of 4,003 COVID-19-positive individuals with a history of influenza, 260 (6.5%) had severe illness from COVID-19, and the overall adjusted odds ratio (aOR) was 1.29 (95% confidence interval 1.04–1.59). Among the 4,003 COVID-19-positive individuals with a history of influenza, severe COVID-19 infection was experienced by 143 of 1,760 (8.1%) with an influenza history within 1 year before the onset of COVID-19, 48 of 1,129 (4.3%) between 1 and 2 years, and 69 of 1,114 (6.2%) between 2 and 3 years before COVID-19 onset, and the aORs were 1.54 (1.20-1.98), 1.19 (0.84-1.70), and 1.00 (0.73-1.37), respectively. In conclusion, individuals who had an influenza infection less than 1 year before COVID-19 infection were at an increased risk of experiencing severe illness from the SARS-CoV-2 infection. To control the public health burden, it is essential that effective public health control measures, which include influenza vaccination, hand washing, cough etiquette, and mask use are in place.

Keywords:

• Influenza
• COVID 19
• SARS-CoV-2
• mortality
• severity
• twindemic

Acknowledgments

This research was supported by a grant of the Korea Health Technology R&D Project through the Korea Health Industry Development Institute (KHIDI), funded by the Ministry of Health & Welfare, Republic of Korea (grant number : HI22C1124) and by the fund of the Biomedical Research Institute, Jeonbuk National University Hospital.

Data availability statement

The datasets analyzed in this study are stored in the Korean National Health Insurance claim-based database. The data will be made available for research on reasonable request and with permission from the National Health Insurance Service (NHIS) (NHIS-2022-1-382).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Ethics statements

This study was approved by the Institutional Review Board (IRB) of Jeonbuk National University Hospital (No. 2020-04-067).

Additional information

Funding

This paper was supported by Bumsuk Academic Research Fund in 2021