Abstract
Background: Influenza virus infection is known to cause excessive hospitalisations and deaths in the elderly during seasonal peaks and pandemics. Methods and materials: We conducted a retrospective, observational cohort study on patients admitted to Tan Tock Seng Hospital from 1 Jan 2010 to 30 Sept 2019. Results: During the study period, 11,728 patients were identified with laboratory-confirmed influenza, influenza A 8,783 (75%) and influenza B 2,945 (25%) respectively. Majority of influenza cases were elderly (68% aged >65 y) with 51% of female, and 8,872 (76%) patients had Charlson co-morbidity score <3. 483 (4%) nosocomial influenza cases (patients had laboratory confirmation >3 d after admission) were identified. Influenza A/B ratio was significantly different between nosocomial and non-nosocomial cases (p = 0.002). Individuals with nosocomial influenza (median 74; range 21–102 y) were older than non-nosocomial cases (median 72; range 13–110 y) (p < 0.001). 49 deaths (10%) occurred among patients with nosocomial influenza infections. During the study period, 754 severely ill patients (who required intensive care and/or died) were identified with overall 6.4% (p = 0.017) among all influenza admissions; 2010 (55, 7.0%), 2011 (49, 6.2%), 2012 (74, 6.5%), 2013 (76, 7.3%), 2014 (84, 6.3%), 2015 (114, 8.4%), 2016 (79, 5.6%), 2017 (115, 6.6%), 2018 (79, 5.7%) and 2019 (29, 4.0%) respectively. On multivariate analysis, factors independently associated with nosocomial influenza cases were co-morbidities of congestive heart failure (AOR 1.47, 95%CI 1.16–1.86), renal disease (AOR 1.76, 95%CI 1.38–2.23), cerebrovascular disease (AOR 1.79, 95%CI 1.36–2.35), any malignancy (AOR 2.19, 95%CI 1.63–2.94) and influenza A (AOR 1.44, 95% 1.14–1.82). In contrast, chronic pulmonary disease (AOR 0.59, 95%CI 0.46–0.76) was less common in nosocomial influenza cases after adjusting for age, gender, ethnicity and admission year. On multivariable analysis, predictors of in-hospital 30 d mortality were age (AOR 1.04, 95%CI 1.03–1.05), Charlson score (AOR 1.18, 95%CI 1.13–1.24) and hospital-acquired influenza (AOR 1.57, 95%CI 1.04–2.37) after adjusting for gender, ethnicity, influenza subtypes and admission year. Conclusion: Annual influenza vaccination should be actively promoted not only for patients with chronic medical conditions but also for healthcare workers who nurse patients especially with low immunity.
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