Background
Coronavirus disease (COVID-19) is an acute infectious disease caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and is characterized by high morbidity and mortality [1, 2]. COVID-19 outbreak began in China in December 2019 and spread rapidly worldwide, with the World Health Organization declaring it a pandemic on March 11, 2020. At present, 4,000,000 confirmed cases of COVID-19 have been detected in more than 200 countries, resulting in more than 280,000 deaths [3], and additional patients with COVID-19 are expected to be cured and discharged over time. Prevention remains the focus for control of COVID-19 [4], but the cured or recovered patients should not be ignored. Currently, little is known about cured COVID-19 patients, and there are still no guidelines regarding the management of these patients. However, it is very important to understand the clinical characteristics of cured patients, especially with respect to re-infection and person-to-person transmission.
During the immune response activated by the infection, IgM levels are usually elevated earlier, indicating recent infection and infectivity, while elevated IgG levels indicate adaptive immunity [5]. However, in patients with COVID-19, the relevance of IgM and IgG antibodies has not been clarified. Researches demonstrated that IgM and IgG could be identified during the early stage of COVID-19, and thus could have a high diagnostic value in patients with acute infection [6-8]. Compared with real-time reverse transcriptase polymerase chain reaction (RT-PCR), the detection of antibodies by ELISA is faster, less expensive, and easier to perform. Therefore, antibody detection might be widely used to assist in the diagnosis of SARS-CoV-2 infection. Till date, no study has evaluated the clinical significance of IgM and IgG detection in terms of re-infection and person-to-person transmission, especially in COVID-19 patients who were cured and discharged home.