Left Atrial Function of COVID-19-recovered Patients During the Surge of Omicron Variants

The goal of this observational study is to retrospectively analyze left atrial function in mild to moderate COVID-19-recovered patients. The main questions it aims to answer are: * Whether left atrial function is involved in patients with mild to moderate COVID-19 after recovery. * What are the factors that may be associated with persistent heart-related symptoms (including chest pain, chest tightness, palpitations, shortness of breath, or postural tachycardia) in patients with mild to moderate COVID-19 ? Participants will undergo transthoracic echocardiography to obtain conventional ultrasound parameters, two-dimensional strain parameters will been obtained through software post-processing, and general clinical data and laboratory test results will been obtained.