Seasonal and Age-Stratified Patterns of Influenza, COVID-19, and RSV in Poland (2024-2026).

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 &#x2265;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 (<i>p</i> = 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 &#x2265;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 &#x2265;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.