Analysis of risk factors between influenza and the occurrence of complicated influenza

Objective:
Influenza is an acute, respiratory viral disease, and may lead to complications such as pneumonia, which presents with significant morbidity and mortality. Previous studies did not have sufficient data regarding the risk factors and complications of the H1N1 flu caused by the influenza A virus subtype H1N1 during the 2009 pandemic in Korea. Therefore, this study was conducted to analyze the characteristics and risk factors of complicated influenza.
Method:
Data were collected from influenza patients who visited a local emergency center in Daegu from January 2017 to December 2019. The study population was divided into three groups: influenza, influenza pneumonia, and influenza with a bacterial infection. The patients’ sex, age, influenza type, comorbidities, vital signs, symptoms, laboratory findings, and clinical outcomes were investigated for the risk analysis of complicated influenza.
Results:
The total number of patients was 574, of which 393 (68.5%) had influenza only, 135 (23.5%) had influenza pneumonia, and 46 (8%) had influenza with a bacterial infection. The odds ratio of complicated influenza was found using multivariate logistic regression analysis; for influenza pneumonia, it was 2.94 for patients aged over 65-years, 3.47 for those with an elevated procalcitonin level, 2.24 for cough, 6.41 for dyspnea, and 3.11 for renal disease. For influenza with bacterial infection, the odds ratio was 2.31 for males, 2.68 for over 80-year-olds, 3.75 for elevated procalcitonin levels, 7.61 for dyspnea, and 3.65 for nursing home residents.
Conclusion:
The risk factors of complicated influenza were advanced age, elevated procalcitonin level, dyspnea, renal disease, and residing in a nursing home.

Key words: Influenza; Pneumonia; Bacterial co-infection; Risk factors