COVID-19 Antibody Tests: An Overview

Abstract

Novel coronavirus (nCoV) first emerged in Hubei province of China in December 2019. The virus initially known as 2019-nCoV was renamed to severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) by the International Committee on Taxonomy of Viruses. The associated disease is known as coronavirus disease 2019 (COVID-19). As the COVID-19 pandemic has unfolded, interest has grown in antibody testing as a way to measure how far the infection has spread and to identify individuals who may be immune. Molecular diagnostic tests like polymerase chain reaction are developed rapidly, however they are not able to fulfill all the requirements of an epidemic reaction. Hence, to complement molecular diagnostic tests, serology tests emerged as a vital aspect of the overall response by confirming the presence of antibodies during the early stage of the infection. Antibody tests help in assessing herd immunity, data about the ongoing phase of infection, identifying potential donors for convalescent plasma therapy, etc. This review currently focuses on giving an overview about the antibody tests in SARS-CoV-2 infections.

INTRODUCTION

Viral diseases continue to emerge possessing a serious threat to public health. Novel coronavirus (nCoV) first emerged in Hubei province of China in December 2019. The virus initially known as 2019-nCoV was renamed to severe acute respiratory syndrome coronavirus-2 (SARS CoV-2) by the International Committee on Taxonomy of Viruses.[1] The SARS-CoV-2 infection also known as coronavirus disease 2019 (COVID-19) was declared a Public Health Emergency of International Concern by the World Health Organization (WHO) on January 30, 2020 and later on as a pandemic outbreak on March 11, 2020.[2] From that point forward, the disease had a rapid global spread, claiming millions of lives worldwide.

On March 16, 2020, the WHO Director-General remarked in his briefing that countries are expected to escalate testing, isolation, and contact tracing, as “the backbone of the response” against this novel pandemic. Later on, he emphasized the importance of increasing the testing capacity of countries to control the spread of the pandemic.[3]

For any infectious disease outburst response, laboratory diagnostic tests play a central role, which is also valid for COVID-19. Various polymerase chain reaction (PCR) tests were hastily developed after the genome release of SARS-CoV-2 and were deployed at the forefront of COVID-19 diagnosis.[4] Molecular diagnostic tests like PCR are developed rapidly, however they are not able to fulfill all the requirements of an epidemic reaction. As the pandemic advanced further and infected millions around the globe, it was concluded that PCR assay exclusively could not meet the different requirements of COVID-19 response, like retrospective contact tracing, detection of asymptomatic infection rate, and appraisal of herd immunity.[5] Hence, to complement molecular diagnostic tests, serology tests emerged as a vital aspect of the overall response by confirming the presence of antibodies during the early stage of the infection. This review currently focuses on giving an overview about the antibody tests in SARS-CoV-2 infections.

MICROBIOLOGY

Coronaviruses belong to the subfamily of Coronavirinae, family Coronaviridae and order Nidovirales according to the International Committee on Taxonomy of Viruses. This subfamily consists of four genera – Alpha, Beta, Gamma, and Delta coronavirus – based on their phylogenetic relationships and genome structures. While the alpha and beta subfamily of viruses infect only mammals, the gamma and delta infect mostly avian species with some exceptions. The current SARS-CoV-2 belongs to the beta group whose structure comprises of crown-like, enveloped, positive single-stranded RNA viruses.[6]

HISTORY

In the past two decades, crossovers of betacoronavirus from animals to humans have been reported in two separate occasions resulting in severe infection with pneumonia-like symptoms. In 2002–2003, a new betacoronavirus thought to originate from bats infected humans through an intermediary host (palm civet cats) in the Guangdong province of China and was designated SARS CoV. It infected 8422 people, causing 916 deaths (mortality rate 11%) in China and Hong Kong before it was contained.[7] A decade later in 2012, the Middle East respiratory syndrome coronavirus, also believed to be of bat origin, emerged in Saudi Arabia with dromedary camels being the intermediate host, affecting 2494 people and causing 858 deaths (fatality rate 34%).[8]

SAMPLES SUITABLE FOR TESTING

As per the Centre for Disease Control and Prevention (CDC) recommendations, the upper respiratory specimen, particularly the nasopharyngeal specimen has been the choice for reverse transcriptase PCR (RT-PCR) molecular assay for COVID-19. The CDC also suggests other specimens such as oropharyngeal, nasal mid-turbinate, anterior nares (nasal swab), or a nasopharyngeal wash/aspirate or nasal aspirate as alternatives whenever nasopharyngeal specimen cannot be obtained. After collection, all the specimens are transported to laboratories for testing in a suitable viral transport medium. The same way the blood sample for serological assay should be collected. All necessary personal protective equipment and strict adherence to infection control and prevention guidelines as stated by the WHO should be followed and monitored by all the health-care professionals involved.[9]

COMMON TESTING METHODS

Conventionally, there are two main types of diagnostic tests for infectious viral diseases – the first type is by detecting the presence of the virus itself establishing current infection status and the second is to detect the presence of antibodies against the virus to understand any prior infection.

1. Methods to find the presence of the virus or tests to detect current/active infection

   • Detection of viral RNA by RT-PCR
   • Antigen tests.

2. Methods to detect prior infection

   • Antibody tests.[10,11]

WHAT CAN TESTING REVEAL

1. Testing someone to identify whether he/she is infected with the SARS-CoV-2 virus at a particular point of time (testing for active/current infection) is very important to:

   • Ensure that infected individuals get appropriate treatment
   • Enable rapid isolation of infected individuals and initiating contact tracing protocol to prevent further spread of infection
   • Identify the number of individuals who are infected on a particular day.

2. Tests performed to identify if a person has been infected with the SARS-CoV-2 virus in the past (testing for prior infection) could be used to:

   • Indicate approximately how many people had infection
   • To prioritize individuals to receive vaccinations, once they are available
   • To find potential donors for convalescent plasma therapy.[10]

ANTIGEN VERSUS ANTIBODY

Antigens are molecules which are able to initiate immune reactions. Distinct features on the surface or epitopes of different antigens elicit particular responses. Y-shaped immunoglobulins (antibodies) which are proteins are produced in response to different antigens by the B cells as a part of the immune mechanism of the body. Each antibody which counteracts specific antigens contains a paratope which identifies a particular epitope on an antigen, binding to them like a lock and key system. This interaction plays an important role in removing antigens from the body, either by direct neutralization or by “tagging” for other arms of the immune system.[11]

UTILIZING ANTIBODIES IN SEROLOGICAL TESTS

When infected with a pathogen, the body's immune response produces antibodies that interact with specific antigen binding to them, thus helping to eliminate the pathogen. This mechanism has been utilized in developing diagnostic tests based on the presence of antibodies and their interactions. An antibody test detects the presence of antibodies, immunoglobulin G, and immunoglobulin M (IgG, IgM) in blood or serum, revealing if an individual has already been exposed to an infection or not.[11]

OVERVIEW OF IMMUNOGLOBULIN G AND IMMUNOGLOBULIN M

Immunoglobulin G

IgG is the most common type of antibody found in blood circulation, which represents approximately 75% of human immunoglobulins (antibodies). It helps in controlling infection by binding to various pathogens such as viruses, bacteria, and fungi and protects the body from infection. They are responsible for long-term immunity after infection or vaccination.

Immunoglobulin M

Produced by B-cells, IgM represents around 10% immunoglobulins in humans. IgM is the largest and the first line of defense in response to an antigen exposure. Hence, they are the early phase immunoglobulins that develop initially during acute infections.[12]

Antibody tests in coronavirus disease 2019

By detecting antibodies in the blood or serum, antibody tests can reveal whether an individual was likely infected with COVID-19 at some time in the past or not. Majority of the tests identify IgM, IgA, and/or IgG against the nucleocapsid protein or viral spike glycoprotein.[13] This detection can be done based on a laboratory test like enzyme-linked immunosorbent assay or chemiluminescent immunoassay, or a point-of-care test based on lateral flow technology.[14] The timing and type of antibody test affect accuracy. Usually, the use of antibody tests to diagnose current infection is limited because of the time lag from infection to the production of antibodies. If the testing is done too early in the course of infection, the test may not detect antibodies because the immune response might still be building up within the body. Hence, it is recommended to perform antibody tests at least 14 days after the onset of symptoms.[15] In case of SARS-CoV-2, recent developments have shown that although IgM and IgG have been shown to be positive as early as the 4^th day after symptom onset, the peak response in case of IgM antibody is around 2 weeks after infection and that of IgG antibody is about 3 weeks. After the peak response IgM antibody level declines reaching lower levels by week 5 and almost disappearing by week 7, whereas IgG levels continue beyond 7 weeks.[16]

Role of antibody tests in coronavirus disease 2019

Antibody tests assist in tracking the spread of disease, which in turn gives a more precise representation of the pandemic. They help in assessing the number of individuals with the previous infection, which is a significant factor in assessing whether herd immunity is obtained or not. Tests which can differentiate between IgM and IgG could provide useful data about the ongoing phase of infection, indicating the duration of infection with SARS-CoV-2. Another benefit of accurate antibody testing is that it can identify potential donors for convalescent plasma therapy. They could also be useful in identifying those who should be prioritized for vaccinations when they become available.[14]

Shortcomings of antibody tests

There are several limitations for an antibody test. Taking into consideration that there are six other coronaviruses known to infect humans, cross-reactivity is a major challenge.[17] If the test is done too early following an infection, a negative test result may be reported. Furthermore, it is not known whether having antibodies to SARS-CoV-2 will protect an individual from reinfection. The level of immunity and how long it will last are also unknown. Recent studies have shown that the level of antibodies starts to drop after 2 months. This implies the fact that the period for which antibody testing can recognize individuals who have been infected is comparatively short. For the above mentioned reasons, the interpretation of antibody test results should be done with caution.[14]

CONCLUSION

Molecular assay such as RT-PCR remains the superior option for etiological diagnosis of the COVID-19 patients, whereas antibody tests can serve as a valuable supplementary aid for the mass screening on a community basis. Both molecular and immunological assay helps to combat the COVID-19 pandemic outbreak, which has affected millions and also the economy worldwide. Appropriate laboratory tests to detect SARS-CoV-2 and specific antibody response are useful tools in guiding patient care and also in making important public health decisions. However, a final interpretation should be made by correlating with different epidemiological and clinical information.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.