The study team established a cohort with a serosurvey in December 2024 and follow-up in March 2025, enrolling 1,454 healthcare workers. Over the season, 50 participants developed COVID-19 and 84 developed influenza. Severity was assessed through functional impairment in daily activities during the acute phase of illness, an approach intended to capture the real-world impact of infection on working and personal life. The researchers used stabilized inverse probability weighting and multinomial logistic regression to estimate relative risk ratios comparing influenza with COVID-19.

Influenza showed higher relative risk ratios than COVID-19 across all severity categories: 1.72 (95% confidence interval, 0.63–4.64) for moderate impairment, 3.50 (1.15–10.7) for severe impairment, and 3.40 (0.87–13.4) for prolonged functional impairment lasting five days or longer. The confidence intervals were wide, reflecting the modest number of cases in each group, but the point estimates consistently indicated that influenza was associated with more disruptive acute illness in this population.

The associations were attenuated after the researchers adjusted for prior infection during the pandemic, a finding that shapes how the results should be read. By the 2024/2025 season, the study population had accumulated very different histories of natural SARS-CoV-2 exposure compared with influenza exposure, and the authors suggest the observed severity contrast may partly reflect these differential natural infection opportunities during the pandemic rather than an intrinsic property of the viruses alone. The serosurvey component also yielded a second finding: higher pre-infection antibody levels against the SARS-CoV-2 nucleocapsid protein were associated with lower severity among participants who went on to develop COVID-19. The study reports an association rather than a demonstrated causal mechanism, so this observation links previous exposure history to the course of subsequent illness without establishing how.

Several limitations frame the conclusions. The cohort consisted of relatively healthy healthcare workers, so the findings cannot be extended to hospitalized patients or to groups at high risk of severe outcomes. The wide confidence intervals mean the severity differences are imprecisely estimated, and the attenuation after adjustment for prior infection leaves the direction of the true contrast uncertain. Functional impairment was the sole severity metric, and the study did not evaluate outcomes such as hospitalization or mortality. Within its scope, the study offers a measured observation for the 2024/2025 season: among working adults infected with either virus, influenza was associated with more severe and longer-lasting disruption of daily activities than COVID-19, possibly shaped by the population’s accumulated exposure history during the pandemic years. The result adds functional impairment to the set of outcomes that comparative studies of respiratory viruses can examine beyond hospitalization and death.

Subject of Research: Comparative acute functional impairment from COVID-19 and influenza in Japanese healthcare workers

Article Title: Functional impairment in daily activities during the acute phase of COVID-19 and influenza among Japanese healthcare workers in the 2024/2025 season

Article References: Yamamoto, S., Mizoue, T., Horii, K., Takeuchi, J. S., Konishi, M., Sugiura, W., & Ohmagari, N. (2026). Functional impairment in daily activities during the acute phase of COVID-19 and influenza among Japanese healthcare workers in the 2024/2025 season. BMC Infectious Diseases. https://doi.org/10.1186/s12879-026-14702-8

Image Credits: AI Generated

DOI: 10.1186/s12879-026-14702-8

Keywords: COVID-19, influenza, healthcare workers, functional impairment, prospective cohort, serosurvey, disease severity, SARS-CoV-2 antibodies, BMC Infectious Diseases, 2024/2025 season, Functional, impairment