Gaining more appreciation on the protective/damaging aspects of anti-SARS-CoV-2 immunity associated with disease severity is of great importance. This study aimed to evaluate the avidity of serum IgG antibodies against SARS-CoV-2 spike (S) and nucleocapsid (N) in hospitalized symptomatic COVID-19 patients and asymptomatic RT-PCR-confirmed SARS-CoV-2 carriers as well as to compare antibody avidities with respect to vaccination status, vaccination dose and reinfection status. Serum levels of anti-S and anti-N IgG were determined using specific ELISA kits. Antibody avidity was determined by urea dissociation assay and expressed as avidity index (AI) value. Despite higher IgG levels in the symptomatic group, AI values of both anti-S and anti-N IgG were significantly lower in this group compared to asymptomatic individuals. In both groups, anti-S AI values were elevated in one-dose and two-dose vaccinees versus unvaccinated subjects, although significant differences were only detected in the symptomatic group. However, anti-N avidity showed no significant difference between the vaccinated and unvaccinated subgroups. Almost all vaccinated patients of different subgroups (based on vaccine type) had higher anti-S IgG avidity, while the statistical significance was detected only between those receiving Sinopharm compared to the unvaccinated subgroup. Also, statistically significant differences in antibody AIs were only found between primarily infected individuals of the two groups. Our findings indicate a key role for anti-SARS-CoV-2 IgG avidity in protection from symptomatic COVID-19 and calls for the incorporation of antibody avidity measurement into the current diagnostic tests to predict effective immunity toward SARS-CoV-2 infection or even for prognostic purposes.

更多地了解与疾病严重程度相关的抗 SARS-CoV-2 免疫的保护/破坏方面非常重要。本研究旨在评估住院有症状的 COVID-19 患者和经 RT-PCR 确诊的无症状 SARS-CoV-2 携带者血清中针对 SARS-CoV-2 刺突 (S) 和核衣壳 (N) 的血清 IgG 抗体的亲和力，以及比较抗体亲和力与疫苗接种状态、疫苗接种剂量和再感染状态的关系。使用特定的 ELISA 试剂盒测定抗 S 和抗 N IgG 的血清水平。通过尿素解离测定测定抗体亲和力并表示为亲和力指数（AI）值。尽管有症状组的 IgG 水平较高，但与无症状个体相比，该组的抗 S 和抗 N IgG AI 值均显着较低。在两组中，与未接种疫苗的受试者相比，接种一剂和两剂疫苗的受试者的抗 SAI 值均升高，但仅在有症状的组中检测到显着差异。然而，抗 N 亲和力在接种疫苗和未接种疫苗的亚组之间没有显示出显着差异。几乎所有不同亚组（基于疫苗类型）的疫苗接种患者都具有较高的抗-S IgG 亲和力，而仅在接受国药控股的患者与未接种疫苗的亚组之间检测到统计学显着性。此外，仅在两组主要感染者之间发现抗体 AI 具有统计学上的显着差异。我们的研究结果表明，抗 SARS-CoV-2 IgG 亲和力在预防有症状的 COVID-19 方面发挥着关键作用，并呼吁将抗体亲和力测量纳入当前的诊断测试中，以预测对 SARS-CoV-2 感染的有效免疫力，甚至用于预后目的。