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

is the (which was not peer-reviewed) The copyright holder for this preprint . https://doi.org/10. 1101 confirmed patients achieve IgG or IgM seroconversion within 20 days after symptoms onset as evidenced by both the cross-section analysis and the follow-up study. This finding indicates that SARS-CoV-2 infection can be ruled out if antibody against SARS-CoV-2 is still undetectable after 20 days of symptoms onset, or after 23 days from exposure (20 days plus a median incubation of 3-day [9] ). A mother and her daughter did not achieve a seroconversion of either IgG or IgM during the hospitalization, emphasizing the necessity of a combination of antibody and RT-PCR tests. Secondly, there is no general rule for the chronological order of IgM and IgG seroconversion for a specific patient, which resembles the conditions in SARS and MERS [2, 11, 12] . This supports the detection of both the IgG and IgM simultaneously rather than the single antibody alone. Thirdly, the confirmation criteria of the serological test for MERS recommended by the WHO can apply to a majority of COVID-19 patients. 82.9% (34/41) of the patients can be diagnosed by the 2 criteria: seroconversion or a ≥ 4-fold increase in the IgG titer in acute (ideally during the first week of illness) and convalescent serum samples [10] . The leading cause for the missing of the patients (12.2% (5/41)) by the criteria is a very early seroconversion (within 7 days or even -4 days of symptoms onset) of IgG in these patients (Fig. 3B, middle panel) . This finding highlights the importance of collecting the first sample as early as possible. On the other hand, we argue that for those patients missed the ideal sampling window, a second serological test several days (for example, 3 days) later that confirms the first positive results would be sufficient to confirm the diagnosis, if with symptoms or chest imaging evidence simultaneously.