Is Antibody test the key to measure immune response to the novel coronavirus?

By Dr Sandeep Sewlikar

Ever since SARS-CoV-2 was declared a public health emergency of international concern in late January 2020, medical professionals and researchers have been urging the need for wide and rapid testing of citizens in order to plan measures that can contain the spread of the virus. During the course of time, real time polymerase chain reaction (RT-PCR) tests and antibody tests have emerged as vital techniques for the global healthcare system to manage the outbreak.

RT-PCR test is the gold standard test for the diagnosis of Covid-19. By detecting viral Ribonucleic Acid (RNA), it is useful in detecting the active infection cases. The Government of India has scaled up the RT-PCR testing significantly over the last few months, which helped to implement the right public health strategy to contain the spread of infection. With early diagnosis of infection, scaling up RT-PCR test helps breaking the transmission chain by isolation and treatment of diagnosed patients.

While researchers focus on developing an effective vaccine for Covid-19, a strong emphasis is laid on understanding the development of antibodies against the virus. Antibody tests mainly used for sero-prevalence studies to understand the percentage of population exposed to the virus in recent past.

What are antibodies and antibody tests?

Antibodies, also called immunoglobulins are protective proteins produced by a person’s immune system in response to antigens, which are essentially foreign substances, during a viral infection. An antibody test measures the presence of antibodies in the blood. The major clinical utility of antibody tests is to assess the people exposed to the virus in the recent past and developed a potential immunity against SARS-CoV-2 infection.

In SARS-CoV-2 infection, antibodies can be developed against 4 structural proteins spike (S), envelope (E), membrane (M), and nucleocapsid (N). Out of these, N and S protein are considered more immunogenic and both suitable targets to antibody assays. In SARS CoV-2, unlike other infectious diseases, detection of IgM does not indicate an acute infection and IgG a past infection. This is because of the variable time of appearance of IgM and IgG in relation to each other (IgG can appear later, at the same time or earlier than IgM) in SARS CoV-2.

There are two types of antibody tests – 1. Rapid antibody tests – It is a point-of-care test and does not require laboratory infrastructure and can be deployed in places away from formal healthcare infrastructure; 2. Laboratory based antibody tests where the blood samples will be tested in the laboratory. The above lab tests are ideal for mass testing programmes.

Differentiating an antibody test from a PCR test


Importance of an antibody test

The clinical utility of antibody tests is multidimensional. Indian Council of Medical Research has recommended that sero-surveys are conducted in 17 segments of “vulnerable” population who are at high risk of contracting infection. A prominent example of this category of people could be health care staff and front line workers, industry workers, security staff, police staff, etc. Antibody tests can be effectively used for:

• Assessing the spread of infection in the community
• Testing effectiveness of vaccines (as intended during clinical trials).
• Prioritization of people to receive vaccines
• Identifying the rate of asymptomatic infection and assessment of mortality attributed to Covid-19 in the community, and identifying the people who are still susceptible to infection.
• Identifying the right donor for convalescent plasma therapy to needy patients.
• As part of the testing algorithm for return to work strategy to minimize the risk of spread of infection.

Elecsys Anti-SARS-CoV-2 is an immunoassay for the in vitro qualitative detection of antibodies (including IgG) to SARS-CoV-2 in human serum and plasma. The assay uses a recombinant protein representing the nucleocapsid (N) antigen for the determination of antibodies against SARS-CoV-2. The test is intended as an aid in the determination of the immune reaction to SARS-CoV-2. Elecsys® Anti-SARS-CoV-2 assay has 99.8% specificity (tested in sample size of 10,453), 99.5% sensitivity and shown no cross reactivity against common coronaviruses.

In SARS CoV-2, it is important to detect antibodies that correlate with neutralization as neutralizing antibodies can potentially be linked with protective immunity. The test format of Elecsys Anti-SARS-CoV-2 (double antigen sandwich assay, a direct antibody format) favors the detection of high affinity, mature antibodies (including IgG). Mature antibodies are likely to correlate with neutralizing antibodies (a subset of mature antibodies), and thus, presumed immunity. Recent independent publications showed that Elecsys Anti SARS-CoV-2 assays have good agreement with neutralization assays.

The Roche antibody assay is widely used in global sero-surveys, mass testing and employee testing protocols in many countries including Singapore (migrant worker testing), UK (health care worker testing) and Switzerland (corporate worker testing). Other countries’ health authorities that are using Roche assay for public testing include US, Indonesia, Myanmar, etc.

Antibody testing is critical to the implementation of effective and efficient public health strategy to minimise the adverse impact of Covid-19 pandemic and should be adopted for important clinical utility cases and to support India’s return-to-work strategy.

The author is the Head- Medical and Scientific Affairs at Roche Diagnostics India.

(DISCLAIMER: The views expressed are solely of the author and ETHealthworld.com does not necessarily subscribe to it. ETHealthworld.com shall not be responsible for any damage caused to any person/organisation directly or indirectly.)

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