Influenza pneumonia among adolescents and adults: a concurrent comparison between influenza A (H1N1) pdm09 and A (H3N2) in the post-pandemic period

Introduction Comparisons of the characteristics between the influenza A (H1N1) pdm09 and common seasonal influenza are important for both clinical management and epidemiological studies. However, the differences between pandemic and seasonal influenza during the post-pandemic period are poorly understood. Objectives The aim of our research was to investigate clinical and immune response differences between patients with influenza A (H1N1) pdm09 pneumonia and seasonal influenza A (H3N2) pneumonia in the post-pandemic period. Methods During the first flu season in post-pandemic period, patients from Beijing Network for Adult Community-Acquired Pneumonia present A (H1N1) pdm09 or A (H3N2) influenza were compared concurrently in the aspects of clinical characteristics and inflammatory profile in acute phase. Result Patients with A (H1N1) pdm09 influenza pneumonia showed a close mean age to A (H3N2) pneumonia (51 ± 20 vs 53 ± 16, mean ± standard deviation, years) but tended to have more underlying diseases (32.8% vs 10%, P = 0.036). Although clinical characteristics were similar, no statistical difference were found in pneumonia severity index (PSI) score or intensive care unit admission rate or mortality, patients in A (H1N1) pdm09 cohort present higher levels of aspartate aminotransferase, lactase dehydrogenase (P = 0.006, 0.018, respectively) in blood and also longer duration of fever than A (H3N2) cohort. Levels of interleukin (IL)-10 and IL-12 (p70) were higher in A (H1N1) pdm09 cohort (P = 0.031, 0.047, respectively). Conclusios During the first post-pandemic flu season, patients with the A (H1N1) pdm09 pneumonia showed similar clinical characteristics but slightly higher disease severity and stronger systemic inflammatory response than A (H3N2) pneumonia.