An indirect in-house immunofluorescent assay was developed in order to assess the serological status of COVID-19 patients in Marseille, France. Performance of IFA was compared to a commercial ELISA IgG kit. We tested 888 RT-qPCR-confirmed COVID-19 patients (1302 serum samples) and 350 controls including 200 sera collected before the pandemic, 64 sera known to be associated with nonspecific serological interference, 36 sera from non-coronavirus pneumonia and 50 sera from patient with other common coronavirus to elicit false-positive serology. Incorporating an inactivated clinical SARS-CoV-2 isolate as the antigen, the specificity of the assay was measured as 100% for IgA titre ≥ 1:200, 98.6% for IgM titre ≥ 1:200 and 96.3% for IgG titre ≥ 1:100 after testing a series of negative controls. IFA presented substantial agreement (86%) with ELISA EUROIMMUN SARS-CoV-2 IgG kit (Cohen’s Kappa = 0.61). The presence of antibodies was then measured at 3% before a 5-day evolution up to 47% after more than 15 days of evolution. We observed that the rates of seropositivity as well as the titre of specific antibodies were both significantly higher in patients with a poor clinical outcome than in patients with a favourable evolution. These data, which have to be integrated into the ongoing understanding of the immunological phase of the infection, suggest that detection anti-SARS-CoV-2 antibodies is useful as a marker associated with COVID-19 severity. The IFA assay reported here is useful for monitoring SARS-CoV-2 exposure at the individual and population levels.

为了评估法国马赛的 COVID-19 患者的血清学状况，开发了一种间接内部免疫荧光测定法。将 IFA 的性能与商业 ELISA IgG 试剂盒进行比较。我们测试了 888 名 RT-qPCR 确诊的 COVID-19 患者（1302 份血清样本）和 350 名对照者，其中包括大流行前收集的 200 份血清、已知与非特异性血清学干扰相关的 64 份血清、来自非冠状病毒肺炎的 36 份血清和来自非冠状病毒肺炎的 50 份血清。患有其他常见冠状病毒的患者，以引发血清学假阳性。采用灭活的临床 SARS-CoV-2 分离株作为抗原，测定的特异性为：IgA 滴度 ≥ 1:200 时为 100%，IgM 滴度 ≥ 1:200 时为 98.6%，IgG 滴度 ≥ 1 时为 96.3%： 100 测试一系列阴性对照后。IFA 与 ELISA EUROIMMUN SARS-CoV-2 IgG 试剂盒基本一致 (86%)（Cohen 的 Kappa = 0.61）。然后在 5 天的进化前测量到抗体的存在率为 3%，在超过 15 天的进化后抗体的存在率高达 47%。我们观察到，临床结果较差的患者的血清阳性率以及特异性抗体滴度均显着高于临床结果良好的患者。这些数据必须纳入对感染免疫阶段的持续了解中，表明检测抗 SARS-CoV-2 抗体可作为与 COVID-19 严重程度相关的标志物。这里报道的 IFA 检测对于监测个体和人群水平的 SARS-CoV-2 暴露很有用。