Age-polarized burden of respiratory syncytial virus and influenza, 1990-2021: global-China evidence for a shift toward older adults.

Background: Lower respiratory infections (LRIs) attributable to respiratory syncytial virus (RSV) and influenza remain major global health threats. In this article, we quantified age-specific and temporal patterns of RSV- and influenza-attributable LRI burden in China and worldwide from 1990 to 2021, and triangulated these findings with real-world institutional testing data. Methods: We extracted deaths and disability-adjusted life years (DALYs) from the Global Burden of Disease (GBD) 2021 study. Age-specific rates and age-standardized mortality rates (ASMRs) were calculated for China and globally, and temporal trends were summarized using estimated annual percent change (EAPC). Analyses were stratified by age group and socio-demographic index (SDI). As complementary evidence, we conducted a retrospective analysis of routine respiratory multi-pathogen testing at The First Affiliated Hospital of Xiamen University (2019-2025), estimating annual and age-stratified (≤5, 5-14, 15-49, 50-70, and ≥70 years) positivity rates for RSV and influenza A/B. Results: In 2021, RSV-attributable LRIs caused 31,525 deaths globally [95% uncertainty interval (UI): 23,348-41,871], largely among children aged <5 years (27,829; 95% UI: 20,300-37,354). Influenza-attributable LRIs caused 98,199 deaths (95% UI: 74,217-126,286), with substantial mortality in both <5 years (27,615; 95% UI: 20,128-36,852) and ≥70 years (45,768; 95% UI: 34,055-60,461). From 1990 to 2019, ASMR in the ≥70-year group increased for RSV globally [EAPC +0.27; 95% confidence interval (CI): 0.07 to 0.47], in contrast to declining trends in younger age groups. Between 2019 and 2021, ASMR declined sharply for both pathogens worldwide (RSV EAPC -41.32; 95% CI: -51.65 to -28.78; influenza EAPC -47.67; 95% CI: -51.67 to -43.34), consistent with broad transmission suppression during the coronavirus disease 2019 (COVID-19) period. Institutional data supported marked perturbations in circulation: among 64,947 tests performed in 2019-2025, overall RSV positivity ranged from 1.9% to 11.3%, peaking in 2021 (11.3%) and rebounding in 2025 (7.9%); positivity was highest in children ≤5 years (peak 34.2% in 2021; 19.4% in 2025), and increased among adults ≥70 years from 1.0% [2023] to 2.2% [2025]. Despite absolute reductions, China's relative global rank in mortality and DALYs worsened in 2021, implying even larger contemporaneous declines elsewhere. Conclusions: RSV and influenza burdens remain concentrated at the extremes of age (<5 years and ≥70 years). A concerning pre-pandemic rise in RSV mortality among older adults was evident, while COVID-19-era controls were associated with a short-term, system-wide downturn; real-world institutional data further suggested suppressed activity followed by rebound and a late increase in older-adult positivity. Age-targeted prevention, particularly for adults ≥70 years, alongside sustained pediatric protection, remains essential.