Edited by: Juarez Antonio Simões Quaresma, Evandro Chagas Institute, Brazil
Reviewed by: Kuldeep Dhama, Indian Veterinary Research Institute (IVRI), India; Leticia Cedillo-Barron, Instituto Politécnico Nacional de México (CINVESTAV), Mexico
This article was submitted to Microbial Immunology, a section of the journal Frontiers in Microbiology
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
The Coronavirus Disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has now become a global pandemic due to its high transmissibility. The unavoidable shortcomings of traditional diagnostic assay, including nucleic acid testing, diverse serological assays characterized by high-throughput and less workload, are playing a more and more crucial role to supplement the nucleic acid test. In this review, we summarize the dynamic change of the specific IgM, IgG, and IgA antibodies against SARS-CoV-2 as well as neutralizing antibodies and discuss the clinical utility and limitations of the different serological assays. SARS-CoV-2, a newly discovered virus, shows some unique pathogenetic and epidemiological characteristics that have not been completely understood so far. Currently, studies about the antibody responses against SARS-CoV-2 and the clinical utility of serological testing are increasing. It’s well suggested that the combination of serological tests and nucleic acid tests can cohesively improve the testing efficiency for identifying COVID-19 suspected patients.
In December 2019, several cases of unknown pneumonia accompanied by respiratory syndromes were reported in Wuhan, Hubei, China (
SARS-CoV-2 is an enveloped, single-stranded RNA virus with oval or round particles that measure about 50–200 nm in diameter (
In this review, we first focus on the current knowledge regarding the humoral response to COVID-19 infection including the profiles of IgM, IgG, IgA, and neutralizing antibodies and then summarize the characteristics of antibodies dynamic change during the whole disease progression. We also discuss the diverse serological assays based on different antigens of SARS-CoV-2 and the limitations of the serodiagnosis. We strongly believe that those serological dynamic assays with high quality assurance system are crucial to facilitate better containment of the epidemic.
Previous studies revealed that the median seroconversion time of specific IgM and IgG against SARS-CoV-2 varies differently, ranging from 5 to 13 days, 11 to 14 days, respectively, after symptom onset (
By contrast, in SARS-CoV infections, specific IgM, IgG, or IgA antibodies against SARS-CoV were all tested negative until at least 3 days after symptoms onset and all positive after at least 19 days (
The seroconversion of specific IgM and IgG antibodies against SARS-CoV-2 mostly turn positive in the second or third week after symptom onset and vary greatly when involved with different specific types of antigens. Consequently, we should pay more attention to the seroconversion time of specific IgM and IgG against SARS-CoV-2 in each different individual with COVID-19 to improve the accuracy and precision of serological assays.
Our previous study conducted in Zhongnan Hospital of Wuhan University, China, showed that both IgM and IgG were first detected at day 4 and the positive rate of IgM and IgG were 11.1% and 3.6%, respectively, in patients of early stages; in intermediate and late stages, the positive rate remained about 75% and increased up to 96.8% for IgM and IgG, respectively (
The seropositive rates of specific IgM, IgG, the combination of IgM and IgG antibody response in patients confirmed with SARS-CoV-2 infection after symptoms onset.
Among most non-ICU patients, total antibodies increased sharply since the first week and elevated during the next 2 weeks; specifically, N-IgM shared a similar dynamic pattern with N-IgG in the first 2 weeks in the same patient, IgM continued to increase until the third week and yet, the level of IgG exceeded IgM in the 2–3 week after onset, indicating that there was an IgM to IgG class switch (
The profiles of virus-specific antibodies vary in different individuals and further studies are needed to explore the uncertainties about the characteristics of immunological response to SARS-CoV-2.
By contrast, in SARS associated infections, nearly all SARS patients show virus-specific IgM and IgG antibody by week 3, which began to decrease in the 3–4 week after illness onset, and IgM remained at a low level in a few weeks, while the concentrations of IgG antibody that is 4∼8 times higher than that of IgM antibody, reaching its peak at 20 days after infection, and then maintained in a high level for a long time (
Current serological tests mainly revolve around the detections of specific IgM, IgG, or total immunoglobins. However, IgA plays a crucial part in mucosal immunity and is considered as the most powerful immunoglobin to fight infectious pathogens in respiratory and digestive systems, which can neutralize SARS-CoV-2 throughout virus entry (
It’s reported that RBD-specific antibodies have greater potency to neutralize infection and thus the RBD of SARS-CoV-2 can serve as an important target for the development of potent and specific nAbs (
Many serological tests based on different antigens of SARS-CoV-2 have been developed in order to confirm suspected patients and to exclude patients infected with other respiratory viruses, thereby facilitating the control of this global pandemic. Some of the most commonly used immunoassays were listed in
The sensitivity and specificity of IgM and IgG detection in different serological tests.
Sensitivity (%) |
Specificity (%) |
Antigen used | References | Advantages | Disadvantages | |||
IgM | IgG | IgM | IgG | |||||
GICA | 88.66* | 90.63* | Recombinant antigen (MK201027) | Rapid, flexible and accurate testing, low cost, and being less time-consuming | False positive, qualitative not quantitative | |||
71.1* | 96.2* | Synthetic antigens of the S, M, and N proteins | ||||||
57.1 | 81.3 | 100 | 100 | Recombinant antigen of new coronavirus | ||||
100* | 93.3* | SARS-CoV-2 NP | ||||||
ELISA | 44.4 | 82.54 | 100 | 100 | Recombinant antigen of new coronavirus | High-throughput, less turn-around time, small sample consumption | Endogenous interference, poor repeatability | |
77.3 | 83.3 | 100 | 95 | The recombinant N protein of SARS-CoV-2 | ||||
70.8 | 92.5 | SARSr-CoV Rp3 nucleocapsid protein (NP) | ||||||
FICA | 98.68 | 98.72 | 93.1 | 100 | The recombinant nucleocapsid protein | High sensitivity and specificity, accurate quantitative detection | Higher requirements for instruments | |
75.6* | 100* | SARS-CoV-2 nucleocapsid protein (NP) | ||||||
87.28 | 90.17 | 94 | 96.72 | N and S1 protein | ||||
CLIA | 78.65 | 91.21 | 97.5 | 97.3 | NA | Easy operation, high sensitivity, large population screening | Poor selectivity, strict external factors needed | |
48.1 | 88.9 | 100 | 90.9 | N and S protein | ||||
96.8 | 96.8 | 92.3 | 99.8 | Highly purified RBD of the S protein | ||||
80 | 90 | 95 | 95 | The combined N and S glycoproteins |
Colloidal gold immunochromatographic assay (GICA) is point-of-care testing (POCT) applied for qualitative analysis of target antigen/antibody, which is a feasible method for the diagnosis of COVID-19 in primary hospitals and laboratories, especially in emergency situations with its advantages of straightforward operation, time-saving and clinically compatible steps, small sample consumption, and easy result interpretation (
In conclusion, GICA is a good diagnostic tool for large population screening and community surveillance for identification of SARS-CoV-2. It’s supplementary to the nucleic acid detection by RT-PCR and provides qualitative results compared to ELISA assay showing quantitative antibody titer (
Enzyme-linked immunosorbent assay (ELISA) is a common biomedical assay that utilizes enzyme-labeled antigens/antibodies to detect specific molecules in specimens and is featured by cost-saving, easy operation (
Recently, a microfluidic ELISA system to detect COVID-19 antibodies on a lab-on-chip platform was described and proposed by a research group. Furthermore, this device first separates plasma from whole blood using a microfluidic device and subsequently performs the detection of antibodies in the separated plasma using a semi-automated on-chip ELISA with high-quality plasma and minimal cell interference (
Fluorescence immunochromatographic assay (FICA) designed for the detection of specific immunoglobulins in blood/serum is based on the combination of immunofluorescence technique and chromatography technology, showing high sensitivity, accurate quantitative detection, and good stability (
It’s reported that the sensitivity and specificity of the FICA with Lanthanide, Eu (III) using the recombinant nucleocapsid protein as antigen were 98.72% and 100% (IgG), and 98.68% and 93.1% (IgM), respectively (
Chemiluminescence immunoassay (CLIA) is based on double-antibodies sandwich immunoassay to detect specific antibodies through amplified signals of chemical luminescence materials. With the features of easy operation and high sensitivity, it is in the central role of the early diagnosis of diseases and large population screening (
Antibody responses to infection take days to weeks to be reliably detectable and the levels of those antibodies decrease over time. It’s reasonable that in early phases of SARS-CoV-2 infections, the levels of specific IgM and IgG antibodies are too low to be detected in the serum or plasma samples leading to false-negative results of serological tests. There is concern that virus-specific antibodies detection may miss cases due to a larger window of time for indirectly detecting SARS-CoV-2 (
The combination of serological tests and nucleotide acid assays is essential to improve the sensitivity and specificity of clinical diagnosis for COVID-19.
It’s widely accepted that cross-reactivity has a great impact on the sensitivity and specificity of the serological tests. A recent study has demonstrated negligible cross-reactivity from human 257 coronavirus, NL63, to SARS-CoV-2 (
Endogenous components in the fingerstick blood or the plasma/serum of venous blood sample can have an effect on the immunoassays of specific antibodies. Notably, endogenous heterophilic antibodies are present in many serum samples to interfere with two-site ELISAs, thereby evoking false-positive signals as well as the human autoantibodies rheumatoid factors (
The outbreak of Coronavirus disease 2019 (COVID-19) caused by SARS-CoV-2 made urgent and necessary the need for diagnostic tests that can identify COVID-19 patients. The dynamic profiles of specific IgM and IgG antibody against SARS-CoV-2 in patients with COVID-19 varies differently. The median time for seroconversion of specific IgM and IgG antibodies is about 9–14 days; however, early seroconversion has been reported at 3–5 days (
Apart from specific IgM and IgG antibodies, the secretory IgA is considered to be powerful in mucosal immunity to eliminate the invasive virus SARS-CoV-2 and the seroconversion of IgA occurred commonly earlier than other antibodies. High levels of neutralizing antibodies are assumed to be associated with the overreaction of innate immune response or strong inflammation and don’t necessarily play the protective role in SARS-CoV-2 infection. The therapeutic efficacy and effects of neutralizing antibodies need to be furthermore elucidated. The dynamic changes in the antibody level of COVID-19 patients can provide more clinical information about the illness progression so that medical staff can cope with COVID-19 patients in a more proper and efficient way. Testing with IgG positive indicates prior infections with the virus and does not necessarily mean protective immunity in infected individuals; what’s more, it’s very likely to be IgG positive while shedding virus as determined by molecular assays due to the timing of infection stages and serum/plasma sampling (
In conclusion, antibody testing can be used as a supplement to the SARS-CoV-2 nucleic acid testing method to confirm and exclude COVID-19. The exact role of the antibody against SARS-CoV-2 in the clinical diagnosis, treatment, and prognosis judgment of COVID-19 needs further study on the pathogenic mechanism of SARS-CoV-2.
YF performed the data analyses and wrote the manuscript. YP contributed to the conception of the study. ZL and YL helped revise the draft and were responsible for the accuracy of this manuscript. All authors contributed to the article and approved the submitted version.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.