15 per cent of samples tested have antibodies, say Thyrocare

Up to 15 per cent of those that underwent antibody tests to know if they have been silently infected in the past are Covid positive. Thyrocare, an Indian lab chain, has conducted tests on more than 60,000 blood samples, and provided the data for 53,000 samples spread over 200 locations in the country.

Thane, for instance, had the highest antibody positivity rates between 17.6 to 47 per cent, while in Bangalore, of all the samples tested, up to 18 to 44.1 per cent were positive. The range is reflective of different pin codes in the cities from which samples were drawn. In certain locations within the same city, the samples with antibodies were higher, while in other locations they were lower. Delhi has between 23.5 to 37.7 per cent positives.

Thyrocare’s founder Velumani A said that the test results are not simple to explain. “If antibodies are present it may offer some support if infection is encountered in future, but there is no guarantee. One must still follow all social norms,” he said.

Velumani said that 80 per cent of the samples are requisitioned for testing by corporate groups for their employees, especially in factory set ups like steel works and mining. “In Jamshedpur and Alibaug, we have tested factory workers in large corporate group set ups. Another 16 per cent demand is coming from housing societies. Another four-odd per cent are individuals who approach us via social media,” Velumani said. In Alibaug for instance, 4.9 per cent of all tested were antibody positive.

Velumani says that after two to three months, antibody levels in an individual’s body come down; however, lymphocytes or white blood cells retain memory of the immunity and may render the person some kind of protection.

Anupam Singh, the Infectious Diseases Consultant at private-run Navin Hospital, a Covid-19-dedicated facility, said that the role of T-cells or lymphocytes in rendering immunity was being explored. “In New York, up to 20 per cent of the population was antibody positive and a surge did not take place after a plateau, even as the protests were occurring. In Delhi, too, this could be thought possible as a parallel,” said Singh.

As a clinician, however, Singh sees symptomatic suspect Covid-19 cases and recommends them the RT-PCR tests that pick up active viral infection. “In case the RT-PCR shows a negative, I suggest them to get a repeat test done. Very rarely do I recommend an antibody test for Covid-19 suspects, only when RT-PCRs are repeatedly negative, but the CT scans show chest infection. While the US agency CDC recommends this, the Indian Council of Medical Research has provided no guidance on this aspect,” said Singh.

Velumani also warns that different companies such as Roche, TransAsia, Omega and Siemens are supplying Enzyme Linked Immunosorbent Assay (ELISA) or Chemiluminescence Assay (CLIA) and the sensitivity of these kits vary. “We should be mindful of variation in tests and rate of false positive results that samples may throw up,” he said.

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