CHENNAI: The recent serosurveillance results show a relatively low prevalence of IgG (Immunoglobulin G) antibodies, but people may still have protection against Covid-19, say experts. This is because people who contracted the disease may have memory-based immunity, which commercial antibody tests don’t detect.A study by the State government found that just 23 per cent of 22,904 samples collected in April showed an exposure to the coronavirus, while a similar study conducted in October last year pegged the exposure at 31 per cent.
Dr Ram Gopalakrishnan, infectious diseases specialist at Apollo Hospitals, says though antibodies wean out, patients may still have cell-based immunity, known as T-cell mediated immunity. “Antibody tests only show whether a person was exposed. T-cell-based immunity is long-lasting but commercial tests can’t detect it,” he explains.Timing is another factor that could affect serosurveillance results. “The first survey was done immediately after the peak, while the second was slightly before the peak,” Dr Gopalakrishnan points out.
Infectious diseases specialist Dr Subramanian Swaminathan, who is also part of the State government’s Covid-19 task force for mucormycosis, says serosurveillance can’t be used to come to a conclusion on immunity, and doesn’t account for vaccine-related immunity. “B-cell immunity, which is antibodies, goes away in three-to-six months. However, T-cell immunity, which may come due to vaccines and also rarely through natural infections, isn’t detected in commercial antibody tests,” he points out.
Former Director of Public Health Dr K Kolandasamy says even though a person has fewer antibodies, memory-based immunity would trigger an immune response to an infection and help a person battle it. “Antibody survey results after the third wave may show a lower prevalence than in the second wave. We cannot say a person’s not immune to the virus based on it,” Dr Kolandasamy asserts, adding that a one-time infection or one-time vaccination reduces the virus spread.