CHENNAI: Following the strategy of Chennai Corporation, which began Covid-19 antibody testing for its staff, the State government has announced it would be conducting sero-surveillance for the rest of the State. It aims at detecting antibody levels against the virus to measure the population’s immunity to its past infection. On the other hand, the Indian Council of Medical Research (ICMR) had given approval for all States to use the rapid antigen-based testing. However, due to concerns prevailing over its efficacy, the State decided not to procure the kits, but instead use the RT-PCR kits, which continues to remain the gold standard.
While the two methods of testing have been doing rounds on the news lately, it has created confusion among the public, as it gives a general perception that both are the same. While both have similar names and provide results in 15-30 minutes, the purpose of both is different. Express explains the differences between various testing methods and explores possibilities of newer antibody tests.
Antigen vs PCR testing
Virologist Dr Jacob John says that antigen tests pick up outside protein coating of the virus when it is present in the body. “Whereas, a PCR test picks up the gene from inside the virus,” says John. He says both the PCR and Antigen tests can pickup the dead virus particles too.”However,why the PCR test remains superior is that it can detect the presence of virus even when the viral load is low. In comparison, the antigen test needs at least1,000 viral particles for it to show positive,” he adds. As per ICMR’s notification, antigen kits have a low sensitivity rate ranging from 50.6 to 84 per cent. Because of this, it has said those who test negative must be tested again using the RT-PCR kit, while those who test positive need not be tested again. This is also the reason why TN will not be using the antigen kits.
Challenges faced
Antibody tests are done mostly by taking blood samples of antibodies that are generated after the infection, John explains. “It detects only the past infection by identifying the antibody Immunoglobulin M, which develops a week after the infection, and Immunoglobulin G, which appears after IgM. Antibody tests are mainly done for sero-surveillance, to identify the spread in the community,” he says. But,there are some challenges with antibody kits as well, say experts.
“A commercial antibody kit only detects IgM and IgG antibodies produced by the B-cells, but it does not detect the antibodies produced by T lymphocyte cells,” says Dr Subramanian Swaminathan,Infectious Diseases specialist at Gleneagles Global Health City. T-cells,which are produced in the thymus gland are part of the human immune response against viral infections. The cells directly target and kill the infected cells.
A recent study from Singapore published in the journal Nature has found that subjects who recovered from Covid-19 had SARS-CoV-2 specific T-cells. The study also examined uninfected individuals, and found the presence of SARS-CoV-2 specific cells in more than 50 per cent of the people, and concluded that it could be the cross-immunity from other coronaviruses. Swaminathan says that as commercial tests do not detect T-cell antibodies, if a person tests negative, it does not mean he is not immune to the virus. Those with a mild infection may test negative, but, they could still be immune, as they may have T-cell antibodies. He goes on to say that if the State is only about to use antibody tests to determine the immunity, it is going to grossly underestimate the immunity strength.
“We need to develop kits for T cell tests and have a look at the results.” Meanwhile, ICMR has capped the price of antigen kits at `450, while an RT-PCR test kit in TN costs up to `3,000. However, the Corporation did not reveal the prices of the antibody kits, which it is using to test its staff. Corporation commissioner G Prakash said the antibody kits are procured using CSR funds and not from the civic body’s budget. “The prices may vary,” he says. The State expects to carry out around 25,000 tests as part of sero-surveillance, and the city corporation is set to carry out another 12,000 tests in its limits.