TN sero survey shows 70% have antibodies against Covid-19

Chennai: Tamil Nadu’s third survey shows a sero-prevalence of 70% across the state, with the districts of Virudhunagar, Tenkasi and Chennai recording the highest exposure to the coronavirus at 88%, 83% and 82%

Chennai: Tamil Nadu’s third survey shows a sero-prevalence of 70% across the state, with the districts of Virudhunagar, Tenkasi and Chennai recording the highest exposure to the coronavirus at 88%, 83% and 82%.

A medic takes swab samples for Covid-19 test of people entering Tamil Nadu from Andhra Pradesh state border at Christianpet check post in Vellore district on April 20, 2021. (PTI)

Serosurvey or Seroprevalence studies are based on analysis of antibodies collected through blood sample collection.

In earlier surveys, the state showed a sero-prevalence of 32% in October-November last year and 29% this April.

The third survey, conducted in July and August just after the second wave began to decline, shows that 70% of the population had IgG antibodies against Covid-19. According to the survey conducted by the Directorate of Public Health and Preventive Medicine, blood samples were drawn from 24,586 people from 827 clusters to be tested using Chemiluminescence based Immuno Assay (CLIA). Each cluster consisted of 30 participants drawn randomly from a village in a rural area or a street in the case of urban regions. As many as 17,090 people were found to have the presence of IgG antibodies against SARS CoV-2.

Coimbatore, Erode and Salem districts in the western region of Tamil Nadu, which had fewer infections during the first wave but were severely affected during the second wave, showed a seropositivity of 71%, 70% and 60%. The lowest seroprevalence was found in Karur district with 51%.

The department says that this is a re-survey as the findings were first released on July 31 which showed a sero prevalence of 66.2%. “We conducted the third survey in July from which we took two extremes -- the highest positivity in five districts and the lowest positivity in five districts -- and we re-did the survey in these 10 districts to check if there are any variations but it’s almost the same,” says Dr V S Selvavinayagam, director of DPH. “But the re-survey confirms our findings and the ground situation, so we have now released the combined results of July to August.”

The survey report comes at a time when Covid-19 infections have declined and there is an increase in vaccinations. “It’s a reflection of the vaccination status. 70% positivity is good but need to go beyond this,” says Dr Selvavinayagam.