In comparison to severe acute respiratory syndrome coronavirus (SARS-CoV), SARS-CoV-2 appears to be more contagious [1], and coronavirus disease 2019 (COVID-19) patients demonstrate varied clinical manifestations distinct from those seen in patients with SARS-CoV and Middle East respiratory syndrome coronavirus infections [2]. Collective results from the clinical and epidemiological observations suggest a distinct viral-host interaction in COVID-19 patients. Profiling of the antibody response during SARS-CoV-2 infection may help improve our understanding of the viral-host interaction and the immunopathological mechanisms of the disease. Studies on humoral responses to infections have been mainly geared toward the production of high-affinity IgG antibodies that efficiently resolve an infection. It has been well recognised, however, that humoral immune response to infection can be a double-edged sword that either serves as a protective mechanism to resolve the infection or aggravates the tissue injury, e.g. the IgG response causes fatal acute lung injury by skewing the inflammation-resolving response in SARS-CoV [3]. In the case of respiratory infection, while IgM and IgG isotypes have been the primary emphasis in characterising immunity, mucosal and systemic IgA responses that may play a critical role in the disease pathogenesis have received much less attention. This study was designed to better understand the timing and patterns of humoral immune responses to SARS-CoV-2 in a cohort of COVID-19 patients and evaluate their relationship with the disease course and severity. 37 patients with COVID-19, with a mean±SD age of 52.3±16.3 years, were enrolled in this study. The enrolled COVID-19 patients consisted of 25 (67.6%) males and 12 (32.4%) females. All patients tested positive for viral nucleic acid of SARS-CoV-2 (Real-Time Fluorescent RT-PCR Kit; BGI, Shenzhen, China). According to the "Guidelines for the Diagnosis and Treatment of Novel Coronavirus (2019-nCoV) Infection" published by the National Health Commission of China, the enrolled COVID-19 patients were categorised into two groups: 20 (54.1%) severe cases and 17 (46.0%) nonsevere cases [4]. The nonsevere group included patients with mild and moderate symptoms who were also required to be admitted to hospital by the COVID-19 control policy in China. The severe group included severe and critically ill patients. Mild patients did not demonstrate abnormal computed tomography (CT) imaging. Moderate patients had fever and/or classical respiratory symptoms, and typical CT images of viral pneumonia. Severe patients met at least one of following additional conditions: 1) shortness of breath with a respiratory rate ⩾30 times•min −1 ; 2) oxygen saturation measured by pulse oximetry (resting state) of ⩽93%; or 3) arterial oxygen tension/inspiratory oxygen tension of ⩽300 mmHg. Critically ill patients met at least one of the extra following conditions in addition to the COVID-19 diagnosis: 1) respiratory failure that required mechanical ventilation; 2) shock; or 3) mu...

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