Abstract
Respiratory syncytial virus (RSV), influenza viruses, and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) remain major causes of seasonal respiratory morbidity, yet their contemporary temporal relationships and age-specific dynamics are insufficiently characterized. We analyzed nationwide Polish surveillance data for the 2024/25 and 2025/26 epidemic seasons, reporting the weekly infection burden per 100,000 population, both overall and by age group (0–4, 5–19, 20–59, and ≥60 years). The seasonal magnitude, peak timing, peak width, normalized temporal trajectories, and age-specific lead–lag patterns were compared. In both seasons, respiratory virus circulation followed the same temporal sequence: COVID-19 activity increased first and peaked in September, followed by influenza in January/February and RSV infections in February/March. Mean weekly burden was lower in 2025/26 for all three infections, by 22.0% for influenza, 19.5% for COVID-19, and 16.0% for RSV, with the between-season difference reaching statistical significance only for influenza (p = 0.0024). Age-stratified analyses likewise showed significantly lower influenza burden in 2025/26 across all age groups, whereas for COVID-19, a significant reduction was observed only among adults aged ≥60 years; no significant between-season differences were found for RSV in any age group. Despite the lower mean influenza burden, the 2025/26 season produced a higher, narrower influenza peak, while the COVID-19 and RSV peaks were lower. Temporal trajectories were highly concordant between seasons for influenza (Spearman’s Rs = 0.975) and RSV infections (Rs = 0.956) and to a lesser extent for COVID-19 (Rs = 0.883). Age-specific temporal trajectories were strongly concordant across age groups, particularly for influenza (pairwise same-week correlations (Rs = 0.972–0.987 in 2024/25 and Rs = 0.963–0.992 in 2025/26). Influenza activity was largely synchronous across age groups, with pediatric peaks occurring approximately 1 week earlier, whereas COVID-19 and, to a lesser extent, RSV showed more consistent pediatric leads relative to adults aged ≥60 years (approximately 1–2 weeks). However, for all three infections, the ascending phase became apparent earlier in children aged 0–4 years than in adults, suggesting that peak timing alone may underestimate the pediatric lead. These findings support integrated, age-stratified surveillance and indicate that preventive strategies, particularly updated COVID-19 vaccination, should be timed to precede the early-autumn rise in transmission.