Distinct kinetics of immunoglobulin isotypes reveal early diagnosis and disease severity of COVID‐19: A 6‐month follow‐up

This study provides a comprehensive dynamic analysis of different antibody isotypes against SARS-CoV-2 and their relation to disease severity or early COVID-19 diagnosis in a large patient cohort over a 6-month follow-up period, which will benefit the early diagnosis and assessment of disease severity for COVID-19. Despite ongoing studies on the kinetics of SARS-CoV-2-specific antibodies, 1–5 it is still insufficient for the comprehensive analysis of long-term kinetics and durability of viral-specific antibody isotypes and their relation to disease severity or early COVID-19 diagnosis. In this study, we eval-uated the dynamics of SARS-CoV-2-specific immunoglob-ulins (A, G, and M) and its correlation to viral clearance and disease severity in a large cohort of COVID-19 patients for 6 months. Besides, IgA and IgG in saliva and bron-choalveolar lavage fluid (BALF) were also assessed for the diagnosis of COVID-19. A patients Shenzhen


Distinct kinetics of immunoglobulin isotypes reveal early diagnosis and disease severity of COVID-19: A 6-month follow-up
To the Editor: This study provides a comprehensive dynamic analysis of different antibody isotypes against SARS-CoV-2 and their relation to disease severity or early COVID-19 diagnosis in a large patient cohort over a 6-month follow-up period, which will benefit the early diagnosis and assessment of disease severity for COVID-19.
Despite ongoing studies on the kinetics of SARS-CoV-2-specific antibodies, [1][2][3][4][5] it is still insufficient for the comprehensive analysis of long-term kinetics and durability of viral-specific antibody isotypes and their relation to disease severity or early COVID-19 diagnosis. In this study, we evaluated the dynamics of SARS-CoV-2-specific immunoglobulins (A, G, and M) and its correlation to viral clearance and disease severity in a large cohort of COVID-19 patients for 6 months. Besides, IgA and IgG in saliva and bronchoalveolar lavage fluid (BALF) were also assessed for the diagnosis of COVID-19.
A total of 506 patients diagnosed with COVID-19 basing on the World Health Organization's interim guidance (WHO 2020) were followed up for 6 months at the Shenzhen Third People's Hospital in this study. All patients were classified into asymptomatic, mild, moderate, and severe groups by the disease severity of COVID-19 (Table 1). IgA, IgG, and IgM against the SARS-CoV-2 spike protein receptor-binding domain (RBD) was measured (Chemiluminescence immunoassay kit, Beijing Wantai Biotech) in 2628 of plasma from 44 asymptomatic carriers, 29 mild, 340 moderate, and 93 severe patients ( Table 1). The overall seroconversion rates of IgA, IgG, and IgM during the follow-up period were 91.93%, 99.59%, and 61.49%, respectively (Table S1).
To understand the correlation between antibody responses and disease severity, the levels of viral-specific IgA, IgG, and IgM were analyzed in four groups, and their kinetics are shown in Figures Figure 1A-C). After this peak, IgG remained at high level across patients including asymptomatic carriers but slowly declined by the sixth month after disease onset ( Figure 1B). IgA also gradually declined after peak ( Figure 1A). In contrast, IgM was present in serum for only 4 months ( Figure 1C). These data indicated that the host antibody responses against spike protein RBD were induced in all disease severity groups and could be kept over 6 months at least after onset of illness. 6,7 Interesting, IgA, IgG, and IgM levels were highest in severe COVID-19 patients as compared to those in other groups (p < .05, Figure 1D-F). Notably, there were more rapid (days after onset) but less robust (peak COI) IgG responses after disease onset in asymptomatic and mild patients compared to moderate and severe patients (p < .05, Figure 1E). Like IgG, IgA was also present later but at higher COI levels in severe patients ( Figure 1D). IgM was present at higher levels in severe COVID-19 patients (median COI: 6.48; IQR: 2.43-14.27) compared to moderate (median COI: 1.40; IQR: 0.50-4.87), mild (median COI: 0.64; IQR: 0.22-1.34), or asymptomatic patients (median COI: 0.40; IQR: 0.27-1.19) ( Figure 1F). Additionally, SARS-CoV-2 was cleared and undetectable 1 month after symptom onset in all groups ( Figure 2D) when SARS-CoV-2specific antibody responses peaked ( Figure 1G). The seroconversion time of IgA, IgG, or IgM was negatively correlated with the lowest Ct value of virus (p < .05 for each, Table S2) and peak levels of total Ig, IgA, IgG, and IgM were risk factors that could be used as independent predictors of disease severity (p < .05 for each, Table 2). These data indicated that the more rapid antibody response occurred more frequently in asymptomatic carriers, while the robustness of antibody was associated with disease severity. 8 However, the seroconversion time of antibodies related to disease severity is controversial and needs further analysis. 2,9,10 To investigate whether the serological assay of different antibody isotypes could improve the early diagnostic power of COVID-19 patients, the seroconversion rates of single or combined antibody isotypes and the detection of viral RNA in the first month after disease onset were analyzed irrespective of disease severity. In each week after disease onset, the seroconversion rate of IgA was significantly higher than IgM (Table 3 and Figure 2A). And the co-seroconversion rate of IgA and IgG (33.11%) was higher than that of IgM and IgG (13.51%) within the first week after disease onset (  Figure 2E). Intriguingly, the level of IgA was similar between serum and saliva (p > .05, Figure 2E), but the level of IgG was much lower in saliva than in serum (p < .0001). Moreover, IgA was the major antibody in saliva, which was much higher than IgG (p < .01, Figure 2E). These data suggest IgA could be more sensitive marker than IgM in the early diagnosis of COVID-19, especially using saliva as easily collected and less-infectious specimen in clinic. In summary, this study comprehensively analyzed the robustness and durability of SARS-CoV-2-specific antibody responses and their relation to disease severity in a large patient cohort over a 6-month follow-up period. There were more robust IgA, IgG, and IgM responses in severe patients, more rapid but less robust IgA and IgG responses in asymptomatic and mild patients. Additionally, IgA was more likely to appear not only in serum but also in saliva during the acute phase of SARS-CoV-2 infec-tion. This study therefore provides new insights for the early diagnosis and prognosis of SARS-CoV-2 infection and improves the understanding of the antibody response in COVID-19.  Abbreviations: +, positive results; IgM+IgG+, co-seroconversion of IgM and IgG; IgA+IgG+, co-seroconversion of IgA and IgG.

A C K N O W L E D G M E N T S
The authors wish to thank the Cell Bank of the Shenzhen Third People's Hospital for bio-samples and services provided. We also thank the clinical staff at Shenzhen Third Peoples' Hospital and all study participants.

C O N F L I C T O F I N T E R E S T
The authors declare that there is no conflict of interest.