Antibody responses to SARS-CoV-2 in COVID-19 patients: the perspective application of serological tests in clinical practice

Quan-xin Long; Hai-jun Deng; Juan Chen; Jieli Hu; Bei-zhong Liu; Pu Liao; Yong Lin; Li-hua Yu; Zhan Mo; Yin-yin Xu; Fang Gong; Gui-cheng Wu; Xian-xiang Zhang; Yao-kai Chen; Zhi-jie Li; Kun Wang; Xiao-li Zhang; Wen-guang Tian; Chang-chun Niu; Qing-jun Yang; Jiang-lin Xiang; Hong-xin Du; Hua-wen Liu; Chunhui Lang; Xiao-he Luo; Shao-bo Wu; Xiao-ping Cui; Zheng Zhou; Jing Wang; Cheng-jun Xue; Xiao-feng Li; Li Wang; Xiao-jun Tang; Yong Zhang; Jing-fu Qiu; Xia-mao Liu; Jin-jing Li; De-chun Zhang; Fan Zhang; Xue-fei Cai; Deqiang Wang; Yuan Hu; Ji-hua Ren; Ni Tang; Ping Liu; Qin Li; Ai-long Huang

(61/63) over the follow-up period. Two patients, a 11-year old girl and her mother, maintained IgG and IgM-negative status during hospitalization, indicating that they did not undergo serological conversion during the observation period. Unfortunately, these two patients were lost for follow-up and the delayed seroconversion cannot be excluded. Serological courses could be followed for 27 patients who were initially seronegative and then underwent a seroconversion during the observation (IgG seroconversion in 19 patients, IgM seroconversion in 20 patients and both seroconversion in 13 patients). All these patients achieved a seroconversion of IgG or IgM within 20 days after symptoms onset. The median day of seroconversion for both lgG and IgM was 13 days (after symptoms onset) (Fig. 3A) . Three types of seroconversion can be observed: synchronous seroconversion of IgG and IgM (10 cases); IgM seroconverted earlier than that of IgG (7 cases); IgM seroconverted later than that of IgG (10 cases). The longitudinal changes of antibody in 6 representative patients of three types were shown in Fig. 3B-D and Fig. S2 . Based on this finding, we recommend that either IgM or IgG seroconversion be used as a confirmation criterion of recent SARS-CoV-2 infection. We tried to identify the factors associated with the different seroconversion types, but found no association between these types and age or gender or hospitalization time (data not shown).