The third sero survey by the Directorate of Public Health and Preventive Medicine (DPH&PM) in July-August 2021 in Tamil Nadu revealed a 70 per cent seropositivity – immunity against a disease due to significant presence of antibodies in the blood serum – due to improved vaccination.

In the earlier State-wide sero surveys conducted by DPH&PM, a seropositivity of 32 per cent was observed in October/November 2020 and 29 per cent in April 2021. The higher percentage in the third sero survey was due to the effort of immunisation coverage. At least 64 per cent of the eligible population have been given one dose of vaccination.

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“Over 5 crores doses have been given so far,” Dr TS Selvavinayagam, DPH&PM, under whose supervision the survey was completed, told BusinessLine .

Blood samples were collected from a total of 24,586 participants and tested for the presence of SARS-CoV-2 IgG antibodies using chemiluminescence-based immuno assay. All the samples were tested at six referral labs of DPH&PM in Chennai, Trichy, Madurai, Coimbatore, Tirunelveli and Salem for the detection of IgG antibodies.

Among the 24,586 samples tested, 17,090 individuals had IgG antibodies against SARS-CoV-2 Virus, and overall sero prevalence was 70 per cent. Highest seropositivity was observed in Virudhunagar district (84 per cent); Tenkasi (83 per cent), Chennai (82 per cent), Madurai (79 per cent) and Theni (76 per cent) districts reported seropositivity above 75 per cent.

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The lowest seropositivity was reported in Karur District (51 per cent). “A total of four districts – Perambalur, Ariyalur, Nilgiris and Karur – reported seropositivity less than 60 per cent,” the survey said.

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