Work

Short-term risks of influenza and COVID-19 following influenza infection: A self-controlled case series study

Year: 2025

Type: article

Abstract: OBJECTIVES: Given the inconsistent existing evidence on viral interference between influenza and COVID-19, this study aimed to investigate the short-term risks of influenza reinfection and COVID-19 infections following an initial influenza episode, in the context of cocirculating influenza and SARS-CoV-2 viruses. METHODS: Using a seasonally adjusted self-controlled case series design, we analyzed surveillance data from Beijing, China, collected between February 2023 and June 2025. The study included individuals infected with influenza who subsequently experienced either influenza reinfection or COVID-19 within 18 months after the initial influenza infection. RESULTS: The study comprised 17,859 individuals with influenza followed by COVID-19 (flu-COVID) and 34,291 with sequential influenza infections (flu-flu). Influenza infection was associated with a transient increase in the risk of COVID-19 within 6 months (incidence rate ratio [IRR] = 1.48, 95% confidence interval [CI]: 1.39-1.59). In contrast, the risk of influenza reinfection was significantly reduced during the same period (IRR = 0.43, 95% CI: 0.39-0.49). Although risk modulation varied across demographic subgroups, results from subgroup and sensitivity analyses were largely consistent. CONCLUSION: Influenza infection may temporarily increase susceptibility to COVID-19 while reducing the risk of influenza reinfection. These findings highlight the importance of targeted surveillance and preventive strategies during influenza peaks to mitigate subsequent COVID-19 burden.

Source: International Journal of Infectious Diseases

DOI: 10.1016/j.ijid.2025.108221