All you wanted to know about India's first ELISA antibody test for COVID-19

ELISA or enzyme-linked immunosorbent assay is a blood test that looks for antibodies. Now, antibodies are very specific proteins that our body makes in response to a specific infection. So, unless you have had the infection (or a vaccine for the infection), your blood would not have the antibodies for it.

NIV’s COVID KAVACH ELISA test looks for IgG antibodies to SARS-CoV-2, the coronavirus that causes COVID-19. IgG or immunoglobulin G is the smallest, most abundant (up to 80% of all antibodies in the blood plasma are IgG) and longest-lasting of the five types of antibodies the human body makes. (The other four antibodies are IgA, IgM, IgE, and IgD.)

How does the test work?

The ELISA test can detect and measure antibodies. Meaning, it can tell you if you have already had a specific infection. Samples in the ELISA test literally change colour if there are antibodies present; darker colouration means more antibodies and no change in colour means no antibodies.

ELISA has been successfully used to test for HIV/AIDS, syphilis, pernicious anaemia, rotavirus and Zika virus among other viral infections and bacterial infections in the past.

Can it be used to diagnose COVID-19?

No, the ELISA test can only tell you if there are antibodies present. It cannot detect active infection.

It is difficult to say at this time whether the presence of SARS-CoV-2 antibodies means you are immune to the disease or that you will never get it again. This is because we know very little about immunity to COVID-19.

There are several factors that researchers are looking into in this regard, including re-infection with another strain of the virus.

However, the presence of antibodies does indicate that you were able to fight the infection in the past. IgG antibodies typically show up two weeks after the infection. Usually, they can be detected even months after someone has recovered from the infection.

How many people could have antibodies to COVID-19?

COVID-19 is, of course, a new viral infection that originated in late 2019. As such, only a small section of the global population that has had the disease and recovered from it has these antibodies. As of 10 June 2020, around 3.4 million people were known to have recovered from the illness. This, however, does not include the unknown number of people who may have had the disease without knowing it (asymptomatic patients).

There could a few important applications for COVID KAVACH ELISA during this pandemic:

• The government has currently outlined its use for doctors, nurses, hospital staff and frontline health workers. These people are at higher risk of contracting the infection. The test can help determine if they have already had the infection and beaten it. This could help them resume work with less anxiety than before.
• The test may also be used to keep the infection in check, especially in the worst-affected districts of the country. As Dr Harsh Vardhan wrote in his tweet on 10 May, this test "will play a critical role in surveillance of the proportion of the population exposed to SARS-CoV-2 coronavirus infection”.
• Although this is not part of the information shared by the Union Health Minister, the test could help to determine who is eligible to donate blood plasma for convalescent plasma therapy (also known as passive antibody therapy). This therapy involves taking blood plasma (with antibodies) from recovered patients and transfusing it into severely ill patients to fight COVID-19. So far, ICMR has only permitted the use of this “experimental” therapy for critically ill patients.

Compared with the rapid antibody test for COVID-19, COVID KAVACH ELISA is supposed to be even more reliable, as it has high “sensitivity and specificity”. This means that the kit can accurately detect the antibodies and determine their number in the blood plasma. While the rapid test can give results in 15-30 minutes, the ELISA test can give results for 90 samples in one go. Both tests give results that can be seen with the naked eye.

Compared with the RT-PCR test for COVID-19 which looks at the genetic material of the virus and can be used to detect active infection, the ELISA antibody test is faster, cheaper. Since the ELISA test samples are not meant for testing active infection, this test also requires fewer bio-safety measures compared with RT-PCR.