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Background: After pandemic, A(H1N1)pdm09 is generally known to be associated with younger adults' infection and greater severity than seasonal A(H3N2) but some inconsistences between recent studies exist. Objectives: We aimed to compare the epidemiology and clinical outcomes of A(H1N1)pdm09 and A(H3N2) to verify and consolidate about the knowledge of known differences of subtypes. Methods: Data were retrospectively collected from the hospital‐based influenza morbidity and mortality surveillance in South Korea in nine tertiary care hospitals, from August 31, 2014, to August 25, 2018. Patients with H1N1pdm09 or H3N2 infection admitted in the emergency room or ward were recruited. Results: A total of 1747 patients had influenza A and were divided into two groups those with A(H1N1)pdm09 (n = 240) and those with A(H3N2) (n = 1507). A(H1N1)pdm09 group had younger age (mean age ± standard deviation 50.0 ± 18.8 in H1N1 vs 53.4 ± 21.1 in H3N2, P =.030), lower influenza vaccination (27.9% vs 43.9%, P <.001) and pneumococcal vaccination rates (41.0% vs 51.9%, P <.001), and fewer underlying diseases (67.5% vs 74.0%, P =.035) than the A(H3N2) group. Influenza A subtypes were not associated with pneumonia risk (adjusted odds ratios [AOR] of A(H1N1)pdm09: 0.7 [95% confidence interval [CI]: 0.4‐1.2, P =.172]) and in‐hospital mortality (hazard ratio (HR) of A(H1N1)pdm09: 1.0 (95% CI: 0.3‐3.1, P =.983)). Influenza vaccination reduced in‐hospital mortality in hospitalized patients (HR: 0.3 (95% CI: 0.1‐0.7), P =.005). Conclusions: A(H1N1)pdm09 infection was more common in younger patients without significant difference in pneumonia risk and in‐hospital mortality between subtypes. Influenza vaccination was associated with reduced in‐hospital mortality. [ABSTRACT FROM AUTHOR]

Background: The influenza virus is reportedly associated with 3‐5 million cases of severe illness and 250 000‐500 000 deaths annually worldwide. Objectives: We investigated the variation of influenza A virus in Korea and examined the association with death. Methods: A total of 13 620 cases were enrolled in the Hospital‐based Influenza Morbidity & Mortality surveillance system in Korea during 2011‐2016. Among these cases, a total of 4725 were diagnosed with influenza using RT‐PCR (influenza A; n = 3696, influenza B; n = 928, co‐infection; n = 101). We used 254 viral sequences from the 3696 influenza A cases for phylogenetic analysis using the BioEdit and MEGA 6.06 programs. Results: We found that the sequences of A/H3N2 in the 2011‐2012 season belong to subgroup 3C.1, whereas the sequences in the 2012‐2013 season pertain to subgroup 3C.2. The sequences in the 2013‐2014 and 2014‐2015 seasons involve subgroups 3C.3a and 3C.2a. The A/H1N1pdm09 subtype belongs to subgroup 6 and contains two clusters. In addition, sequence analysis confirmed the several substitutions of internal genes and gene substitutions associated with drug resistance (I222V in NA and S31N in M2) in the fatal cases. While statistical analysis found no significant associations between genetic differences in the viruses and mortality, mortality was associated with certain host factors, such as chronic lung disease. Conclusions: In conclusion, influenza A virus clade changes occurred in Korea during the 2011‐2016 seasons. These data, along with antigenic analysis, can aid in selecting effective vaccine strains. We confirmed that fatality in influenza A cases was related to underlying patient diseases, such as chronic lung disease, and further studies are needed to confirm associations between mortality and viral genetic substitutions. [ABSTRACT FROM AUTHOR]

The Hospital-based Influenza Morbidity and Mortality ( HIMM) surveillance system is an emergency room ( ER)-based influenza surveillance system in Korea that was established in 2011. The system was established under the assumption that integrated clinical and virologic surveillance could be performed rapidly and easily at seven tertiary hospitals' ER. Here, we assessed the correlation between data generated from the HIMM surveillance system and the Korean national influenza surveillance systems during the 2011-2012 influenza season using cross-correlation analysis and found strong correlations. Rapid antigen-test-based HIMM surveillance would predict the start of influenza epidemic earlier than pre-existing influenza-like-illness-based surveillance. [ABSTRACT FROM AUTHOR]