Unveiling the role of preceding seasonal influenza in the development of bacteremic pneumococcal pneumonia in older adults before the COVID-19 pandemic in Japan.

Objective: We aimed to investigate the impact of preceding seasonal influenza on the clinical characteristics of adult patients with invasive pneumococcal disease (IPD) in Japan.

Methods: Data for 1722 adult patients with IPD were analyzed before (2017-2019) and during the COVID-19 pandemic (2020-2022).

Results: The seasonal influenza epidemic disappeared soon after the emergence of the pandemic. Compared with that before the pandemic (66.7%), we observed a lower bacteremic pneumonia proportion in patients with IPD during the pandemic (55.6%). The clinical presentations of IPD cases significantly differed between those with and without preceding influenza. The proportion of bacteremic pneumonia was higher in IPD patients with preceding influenza than in those without in both younger (44.9% vs 84.2%) and older adults (65.5% vs 87.0%) before the pandemic. The case fatality rate was significantly higher in IPD patients with preceding influenza (28.3%) than in those without (15.3%) in older adults before the pandemic (P = 0.020). Male and aging are high risk factors for death in older patients with IPD who had preceding influenza.

Conclusion: Our study reveals that preceding seasonal influenza plays a role in the development of bacteremic pneumococcal pneumonia, increasing the risk of death in older adults.