Background/Objectives: Respiratory syncytial virus (RSV) is a leading cause of respiratory infections across all age groups, with the greatest burden observed in young children and older adults. The COVID-19 pandemic significantly disrupted RSV circulation, resulting in an immunity gap and altered transmission dynamics. This study aimed to assess the seroprevalence of anti-RSV IgG antibodies in the Polish population during the 2023/2024 epidemic season. To our knowledge, this is the first study to characterize RSV seroprevalence at the population level in Poland. Methods: A total of 700 serum samples from individuals across different age groups were analyzed using a commercial assay to detect anti-RSV IgG antibodies. Seroprevalence and antibody levels, expressed as the index of positivity (IP), were examined by age and sex. Results: The overall seroprevalence of anti-RSV IgG antibodies was 91.4%. Antibody positivity increased markedly from 35.5% in infants aged 0–1 years to over 90% in children aged 4–5 years, reaching nearly universal levels in older age groups, including 99.1% in adults aged ≥60 years. Median IP values also rose with age, peaking in individuals aged ≥60 years. No significant differences in seroprevalence were observed between sexes, though older men showed slightly higher median IP values, potentially reflecting greater cumulative RSV exposure. Conclusions: This study provides key insights into the post-pandemic landscape of RSV immunity in Poland. The high seroprevalence across most age groups underscores widespread prior exposure, while the lower rates in infants highlight a continued vulnerability. These findings support the development and implementation of targeted immunization strategies, particularly for infants and older adults.

Background: Depression is a common psychiatric disorder that may be caused by dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis. The glucocorticoid receptor (GR) plays a significant role in regulating this axis. One negative regulator of GR action, previously associated with depressive behavior, is the overexpression of FK506-binding protein 5 (FKBP5), which may be regulated by microRNAs, including miR-511-5p. Objectives: In a rat model of depression, we aimed to investigate the expression of Fkbp5 and its regulator, miRNA-511-5p, during short- and long-term lithium treatment in four brain regions: the hypothalamus, hippocampus, pituitary, and frontal cortex. Methods: We used a rat model of depression induced by chronic mild stress (CMS) to assess if short- and long-term lithium treatment (7 and 42 days) influences Fkbp5 expression in the brain. We also assessed the effects of lithium treatment on the blood levels of corticosterone in CMS-exposed rats as compared to control groups. The changes in the expression of Fkbp5 were assessed by qPCR and Western blot. The expression of rno-miR-511-5p was assessed using qPCR. Statistical analysis was conducted in GraphPad Prism 9. Results: We found that long-term lithium treatment increased the expression of the FKBP5 protein in the pituitary gland in the lithium-treated rats compared to the control group. We also observed significant changes in Fkbp5 mRNA levels between CMS-exposed rats compared to the control animals, without significant changes in mRNA levels observed during short- and long-term lithium treatment in any brain region. We found no expression of rno-miR-511-5p in the analyzed brain regions. The corticosterone levels were significantly higher in CMS-exposed rats compared to the control, with no significant changes found between lithium-treated and control rats. Conclusions: FKBP5 protein expression in the brain may be regulated by long-term lithium treatment, thus affecting GR signaling in the rat depression model.

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.