The seroprevalence and kinetics of IgM and IgG in the progression of COVID-19

Coronavirus disease 2019 (COVID-19) has caused a great pandemic worldwide. As of October 12,020, more than 3 billion cases had been diagnosed with COVID-19, and the mortality rate was about 3.0% according to World Health Organization (WHO) reports. The challenges of this epidemics include treatment, avoiding viral transmission. The Chinese diagnostic and therapeutic guidelines of COVID-19 have been updated 7 times [1]. More detailed information on COVID-19 has been uncovered by clinical and basic research.

One of the dilemmas in the treatment of COVID-19 is the relatively high rate of false-negative results using nucleic acid tests as the diagnostic method. The reasons for this include several aspects, including a low viral concentration in the upper respiratory tract, unstandardized sample collection methods, various gene application performances, and a decrease in viral load one week after disease onset [2, 3].

Since an immune reaction is involved in COVID-19 progression, serological assays have been developed and put into practice in many countries [4]. Antibody tests have been confirmed as a good supplement for nucleic acid tests. They can be used as an immunity passport or proof of a previous infection, an asymptomatic infection or immunization. However, there are still many challenges and knowledge gaps in the clinical applications of antibodies in COVID-19 [5]. including the performances of various SARS-CoV-2 antibody products, the variable prevalences of antibodies in different regions, and the interpretation of positive results in various clinical stages. Previous studies on SARS-CoV-2 antibody tests in China were mostly conducted in a single centre or restricted to one province. Our study collected data from four epidemic cities during the outbreak stage of COVID-19 in China to better understand the significance of SARS-CoV-2 serological tests.

This is the first multicentre study of antibodies against SARS-COV-2 from four outbreak areas in China. We found a good clinical coincidence rate of the antibody tests, an association between the clinical classification and the concentration of antibodies, and the kinetics of the antibodies, which could improve our understanding in the immune response after patients are infected with SARS-CoV-2. In this study, we investigated the seroprevalence of SARS-Cov-2 IgG and IgM as well as the kinetics of the antibody response in COVID-19 from four epidemic regions of China in early 2020 [6]. In the cross-sectional study, 91.9 and 92.3% of patients with confirmed SARS-CoV-2 infection tested IgG and IgM positive, respectively. 2.1% of 336 suspected patients tested IgG positive, and 5.4% tested IgM positive. In the cohort study, the peaks of IgM and IgG were reached on the 10th and 20th days, respectively, after symptom onset. The seroprevalence of SARS-CoV-2 IgG and IgM increased along with the clinical classification and treatment time delay.

There has been an urgent need for novel in-vitro diagnostic products. The sensitivity and specificity of these products ranged 88–100% and 75–100%, respectively [7‐9]. Sensitive and stable CLIA was used as a measurement method in our study. Figure 1a shows that the positive antibody rate increased along with the symptom onset intervals. The seroprevalence of SARS-CoV-2 IgM and IgG on ≤7 days since onset of symptoms were 81 and 83%, respectively. The seroprevalence of both antibodies raised to 95% at 2 weeks since symptom onset [10].

Approximately 8% of COVID-19 patients tested negative for IgM or IgG. We speculated three explanations which might contribute to the negative results for confirmed COVID-19 patients. First, the heterogeneity of testing times from the onset of the disease are an important factor [11, 12]. Previous study showed that antibodies elicited by SARS-CoV-2 develop three days after symptom onset or one week after infection with SARS-CoV-2 [13]. In our study, the average time interval from onset to sampling was 3 days in Beijing, followed by Tianjin, Shijiazhuang, and Wuhan in an increasing order of average time interval (Table 1). Therefore, the seroprevalences of IgM and IgG antibodies in Shijiazhuang and Tianjin were higher than those in Beijing and Wuhan. Therefore, antibody testing during the window phase of COVID-19 progression could lead to false-negative results [14, 15]. Secondly, individual differences in the immune response are also a contributing factor. Some COVID-19 patients were negative for SARS-CoV-2 IgM and IgG from onset to recovery [16], which indicates that innate immunity could clear the virus without adaptive immunity and these patients might not produce detectable antibodies against SARS-CoV-2 [15, 16]. Additionally, the sample size of COVID-19 patients was limited in Beijing and Wuhan, which might lead to underestimation of the seroprevalence [17].

On the other hand, 2.1 and 5.4% of patients were found SARS-CoV-2 IgG and IgM positive in suspected patients. Many causes of false-positive results have been reported, including autoimmune disease, cancer, drug usage, and other infections [18, 19]. Therefore, antibody tests are recommended in combination with nucleic acid tests for the diagnosis and treatment of COVID-19.

The association of SARS-CoV-2 antibody concentration, positive rate, and clinical stage was explored in our study. Although there was no statistical significance due to limited number of patients included in this study, SARS-CoV-2 IgM and IgG antibody concentrations and positive rates in severe cases were distinctively higher than those in milder cases (Table 2). Our study further confirmed the findings from the previous study. Long et al. and Qu et al. indicated that critical COVID-19 patients had higher IgM and IgG antibody responses than non-critical patients [20, 21] due to a high level of viral load or inflammatory storm in severe or critical cases [22]. In addition, we found that the mean ages of COVID-19 patients with severe cases were older than those with milder cases, although the difference was not statistically significant. Many studies have reported that patients who died were generally older than survivors in critical cases of COVID-19 [23‐25], especially among patients with comorbid diseases, including hypertension, coronary artery disease, and diabetes [25, 26]. Age is one of the risk factors for susceptibility and poor prognosis of COVID-19 [27].

The kinetics of SARS-CoV-2 IgM and IgG in COVID-19 patients are shown in Fig. 2. The IgM antibody concentration reached a peak 10 days earlier than the IgG antibody concentration. The SARS-CoV-2 IgG antibodies maintained an upward trend after 20 days. Andrea et al. reported that IgM antibody levels peaked at 10–12 days and significantly declined after 18 days [28] which was similar to our study. IgG against COVID-19 has been reported to persist over seven weeks [11]. Some studies showed that COVID-19 patients with high IgG titres might produce neutralizing antibody activity, clearing the virus [29, 30]. Wang et al. reported a moderate correlation between anti–SARS-CoV-2 spike protein IgG levels and neutralization titres in COVID-19 patient plasma [5]. In contrast, some studies observed higher levels of anti-RBD IgG antibodies from COVID-19 patients that did not contribute to neutralization. They suggest that anti-RBD IgM and IgA also contribute to neutralization [31, 32]. Since the virus-neutralizing antibody titre was determined by the virus infection inhibition rate, the content of neutralizing antibodies in the serum was found to be complex and is being recognized gradually [33]. The detection antibodies in commercial reagents usually target spike and/or nucleocapsid proteins and may not distinguish among different immunogenic regions of the spike protein of SARS-CoV-2 [34]. Therefore, predicting whether serum with positive antibodies is protective or therapeutic should be approached with caution.

Nevertheless, there were some limitations in our study. First, the limited sample size and clinical information in some regions restricted more analysis to perform. Additionally, individuals with asymptomatic SARS-CoV-2 infection were not included in our studies, so information from those patients was lacking.

In conclusion, we demonstrated the seroprevalence of SARS-CoV-2 IgM and IgG antibody and antibody titre alterations in COVID-19 patients, which could help in better interpreting the antibody testing results during COVID-19 progression.