Clinical Manifestations of Patients with Influenza Differ by Age: A Prospective, Multi-centered Study in the Setouchi Marine Area

Influenza potentially has a high mortality rate when it affects the elderly. We aimed to examine the differences in clinical manifestations in patients with influenza according to their age. This multicenter prospective study was performed in six medical institutions in Okayama and Kagawa prefectures (Japan). Between December 1, 2019 and March 31, 2020, we collected data on adult patients diagnosed with influenza type A, who were stratified into younger (20-49 years), middle-aged (50-64 years), and older groups (>= 65 years). We compared the presence or absence of fever, respiratory symptoms, and extrapulmonary symptoms according to age group. In total, 203 patients (113, younger; 51, middle-aged; and 39, older) were eligible for the analysis. The maximum body temperature and temperature at first physician visit in the older group were significantly lower than those in the younger group. The incidence of respiratory symptoms was not different among the three groups. Chills, muscle pain, and arthralgia as systemic symptoms were noted significantly more frequently in the younger (80.9%) and middle-aged (75.5%) groups than in the older group (51.3%) (p = 0.002). Fever and systemic symptoms were less likely to appear in older patients, possibly resulting in the delaying of hospital visits among older adults.