Shifting Seasonality of RSV and Influenza-Like Illness (ILI) in the Post-COVID Era: A Narrative Literature Review of Global and U.S. Trends Abstract Background: Before the COVID-19 pandemic, Respiratory Syncytial Virus (RSV) and Influenza-Like Illnesses (ILI) followed predictable winter peaks, guiding healthcare preparedness. However, the extensive non-pharmaceutical interventions (NPIs) implemented globally to curb SARS-CoV-2 profoundly altered the epidemiology and seasonality of these respiratory viruses. Objective: This review examines the shifts in the timing, intensity, and demographic distribution of RSV and ILI from 2020 to 2024, comparing global and U.S. trends and exploring implications for surveillance and vaccination strategies. Methods: A systematic literature review was conducted following PRISMA 2020 guidelines. PubMed, Scopus, Cochrane Library, and WHO Global Health Library were searched for peer-reviewed articles published in English between March 2020 and November 2024. Eligible studies described RSV or ILI epidemiology before and after COVID-19. Findings were synthesized narratively due to heterogeneity in study designs. Results: NPIs led to a near disappearance of RSV and influenza during 2020–2021, followed by atypical, off-season resurgences as restrictions lifted. In the U.S., RSV activity reappeared unusually early in mid-2021, peaking in summer rather than winter, with similar patterns observed in Europe and Australia. The disruption produced an “immune debt,” marked by older age at hospitalization and higher post-pandemic infection intensity. While seasonality began reverting toward prepandemic norms by 2023, considerable variability persisted. Conclusion: COVID-19-related NPIs caused a historic reshaping of respiratory virus circulation, highlighting the fragility of viral seasonality. Continuous, year-round surveillance and flexible immunization strategies such as maternal RSV vaccination and monoclonal antibody prophylaxis are essential to adapt to evolving epidemic patterns. Further research should explore viral interactions, immunity gaps, and the long-term reestablishment of stable respiratory seasonality.