Seasonal Distribution of Viral Pneumonia After COVID-19 and the Role of Hematological Markers in Assessing Pneumonia Severity: A Case–Control Study

Background: Various studies have shown that viral pneumonia pathogens display distinct inflammatory profiles, and hematological indices, such as the Neutrophil/Lymphocyte Ratio (NLR), Lymphocyte/Monocyte Ratio (LMR), and Platelet/Lymphocyte Ratio (PLR), can serve as accessible markers of disease severity. Moreover, the seasonal distribution of respiratory viruses appears to have shifted during the COVID-19 pandemic. Methods: This retrospective case–control study was conducted on patients diagnosed with PCR-confirmed viral pneumonia in the emergency department of a tertiary care center between 1 January and 31 December 2024. The control group comprised age- and sex-matched individuals without viral pneumonia. Subjects with comorbidities or ongoing treatments potentially affecting hematological indices were excluded. Seasonal distribution of viral pathogens was recorded. Hematological and inflammatory parameters at admission—including neutrophil-to-lymphocyte ratio (NLR), lymphocyte-to-monocyte ratio (LMR), and platelet-to-lymphocyte ratio (PLR)—were evaluated. The associations between these biomarkers, Pneumonia Severity Index (PSI) scores, and hospitalization status were statistically analyzed. Results: In this study, it was determined that Influenza A/B was more common in winter (67.3%) and SARS-CoV-2 in summer (70.7%). The relationship between the Pneumonia Severity Index and hemogram parameters was examined in determining the severity of pneumonia. In SARS-CoV-2, leukocyte and neutrophil counts were positively correlated (R: 0.392, p: 0.003; R: 0.466, p: <0.001), while in Influenza A/B, lymphocyte, platelet, and monocyte counts showed a negative correlation (R: −0.402, p: 0.005; R: −0.331, p: 0.021; R: −0.327, p: 0.023). Correlations were found between inflammation parameters and the Pneumonia Severity Index, except for the Lymphocyte/Monocyte Ratio, between SARS-CoV-2 and Influenza A/B (p < 0.05). Conclusions: The seasonal distribution of viral pneumonia pathogens has been revealed following the COVID-19 pandemic. Due to differences in inflammation patterns in viral infections, different leukocyte subgroups have been suggested as biomarkers.