Severity and outcomes of adult respiratory syncytial virus inpatient compared with influenza: observational study from Japan.

Background: Respiratory syncytial virus (RSV) significantly impacts not only children but also adults. However, knowledge of the severity and outcomes among adult RSV inpatients is still limited.

Objectives: To clarify the short- and long-term health threats associated with adult RSV infections.

Methods: This retrospective observational study included 56,980 adult inpatients aged 18 years and older due to RSV or influenza infection between April 2010 and March 2022. After inverse probability weighting adjustment, we used Poisson's regression to estimate the risk of outcomes.

Results: The RSV group had a higher risk of requiring mechanical ventilation during hospitalization compared to the influenza group (9.7% vs. 7.0%; risk ratio (RR), 1.35; 95% confidence interval (CI), 1.08-1.67). In-hospital mortality was comparable between RSV and influenza groups (7.5% vs. 6.6%; RR, 1.05; 95% CI, 0.82-1.34). RSV group was associated with increased risk of readmission within 1 year after surviving discharge (34.0% vs. 28.9%; RR, 1.19; 95% CI, 1.07-1.32) and all-cause mortality within 1 year of admission (12.9% vs. 10.3%; RR, 1.17; 95% CI, 1.02-1.36). In the age-stratified analysis, the RSV group aged 60 years and older had a higher risk than the influenza group for in-hospital death, readmission and all-cause mortality within one year.

Conclusions: RSV infections demonstrated comparable or greater health threats than influenza infections not only during hospitalization but also in long-term outcomes. The findings underscore the threat of RSV in adults, the impact on healthcare systems and the need for continued development of public health counter measures against RSV.