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Int J Infect Dis
. 2026 Jul 7:108958.
doi: 10.1016/j.ijid.2026.108958. Online ahead of print.
Concurrent comparison of severity of influenza and COVID-19 among hospitalized patients in Hong Kong: a target trial emulation on observational healthcare data
Celine Sze Ling Chui 1 , Jack Chi-Him Lau 2 , Min Fan 2 , Brian Pui-Chun Chan 3 , Wang-Chun Kwok 4 , Chun-Ka Wong 5 , Kelvin Kai Wang To 6
Affiliations
Background: Influenza virus (IV) and SARS-CoV-2 are significant causes of hospitalization. Earlier studies showed more severe disease for SARS-CoV-2 infections, but many were compared with historical influenza cohorts and were conducted before the milder Omicron variant emerged. We aimed to compare the severity of SARS-CoV-2 infection with a concurrent cohort of IV infection among hospitalized patients after the emergence of Omicron variant in Hong Kong SAR China (HK).
Methods: This is a target trial emulation study based on the public electronic health database in HK. Adults aged ≥18 years hospitalized and with laboratory confirmed SARS-CoV-2 or IV infection between 1st January 2023 and 31st January 2024 were included. Pooled logistic regression was used to explore the risk of composite outcome (30-day mortality, mechanical ventilation, ICU admission).
Results: A total of 47,956 monoinfection cases were identified, including 32,523 SARS-CoV-2 monoinfection, 15,103 IV monoinfection, and 330 SARS-CoV-2/IV coinfection. Overall, the risks of composite outcome (HR=1.14 [95%CI 1.06-1.23]), 30-day mortality (HR=1.20 [95%CI 1.09-1.32]) and ICU admission (HR=1.21 [95%CI 1.06-1.38]) were higher for SARS-CoV-2 than IV infection. Subgroup analysis showed that male SARS-CoV-2 patients were at higher risk of composite outcome (HR=1.19 [95%CI 1.08-1.31]), 30-day mortality (HR=1.26 [95% CI 1.11-1.44]) and ICU admission (HR=1.2 [95%CI 1.02-1.4]) than male IV patients. Post-hoc analysis showed that SARS-CoV-2/IV coinfection was associated with an increased risk of 30-day mortality than SARS-CoV-2 monoinfection (HR=2.11 [95%CI 1.58-2.81]) or influenza monoinfection (HR=3.45 [95%CI 2.19-5.42]).
Conclusion: SARS-CoV-2 Omicron infection was associated with more severe outcomes than IV monoinfection, and coinfection more severe than monoinfection among hospitalized patients. Our findings have important implications for healthcare policies, including vaccine recommendation for SARS-CoV-2.
Keywords: COVID-19; SARS-CoV-2; coinfection; influenza.
. 2026 Jul 7:108958.
doi: 10.1016/j.ijid.2026.108958. Online ahead of print.
Concurrent comparison of severity of influenza and COVID-19 among hospitalized patients in Hong Kong: a target trial emulation on observational healthcare data
Celine Sze Ling Chui 1 , Jack Chi-Him Lau 2 , Min Fan 2 , Brian Pui-Chun Chan 3 , Wang-Chun Kwok 4 , Chun-Ka Wong 5 , Kelvin Kai Wang To 6
Affiliations
- PMID: 42413685
- DOI: 10.1016/j.ijid.2026.108958
Background: Influenza virus (IV) and SARS-CoV-2 are significant causes of hospitalization. Earlier studies showed more severe disease for SARS-CoV-2 infections, but many were compared with historical influenza cohorts and were conducted before the milder Omicron variant emerged. We aimed to compare the severity of SARS-CoV-2 infection with a concurrent cohort of IV infection among hospitalized patients after the emergence of Omicron variant in Hong Kong SAR China (HK).
Methods: This is a target trial emulation study based on the public electronic health database in HK. Adults aged ≥18 years hospitalized and with laboratory confirmed SARS-CoV-2 or IV infection between 1st January 2023 and 31st January 2024 were included. Pooled logistic regression was used to explore the risk of composite outcome (30-day mortality, mechanical ventilation, ICU admission).
Results: A total of 47,956 monoinfection cases were identified, including 32,523 SARS-CoV-2 monoinfection, 15,103 IV monoinfection, and 330 SARS-CoV-2/IV coinfection. Overall, the risks of composite outcome (HR=1.14 [95%CI 1.06-1.23]), 30-day mortality (HR=1.20 [95%CI 1.09-1.32]) and ICU admission (HR=1.21 [95%CI 1.06-1.38]) were higher for SARS-CoV-2 than IV infection. Subgroup analysis showed that male SARS-CoV-2 patients were at higher risk of composite outcome (HR=1.19 [95%CI 1.08-1.31]), 30-day mortality (HR=1.26 [95% CI 1.11-1.44]) and ICU admission (HR=1.2 [95%CI 1.02-1.4]) than male IV patients. Post-hoc analysis showed that SARS-CoV-2/IV coinfection was associated with an increased risk of 30-day mortality than SARS-CoV-2 monoinfection (HR=2.11 [95%CI 1.58-2.81]) or influenza monoinfection (HR=3.45 [95%CI 2.19-5.42]).
Conclusion: SARS-CoV-2 Omicron infection was associated with more severe outcomes than IV monoinfection, and coinfection more severe than monoinfection among hospitalized patients. Our findings have important implications for healthcare policies, including vaccine recommendation for SARS-CoV-2.
Keywords: COVID-19; SARS-CoV-2; coinfection; influenza.