Second TN sero-survey says 23% of population had antibodies as of April

The results show a decline from 31% of people who showed traces of antibodies in the first phase of the survey held in October-November 2020.
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The results of the second state-wide cross sectional sero-survey conducted by the Tamil Nadu Health Department in April were published on Sunday, with the state recording an overall sero prevalence of 23%, as opposed to 31% in the first survey held last year. The Directorate of Public Health and Preventive Medicine (DPH&PM) conducted a state-wide cross sectional survey in April this year in all districts excluding Chennai. For the survey, 756 clusters were picked, covering as many as 22,904 samples. The survey held under the supervision of DPH&PM Director TS Selvavinayagam consisted of participants randomly picked from rural villages and cities.

Sero surveys are usually conducted to understand the spread of a virus in the general population. Experts have said that for a virus like coronavirus, herd immunity is estimated to kick in somewhere around 60-70% seroprevalence. According to a press note released by the Health Department, the blood samples collected from 22,904 participants were tested for the presence of SARS CoV-2 antibodies at labs in Chennai, Salem, Trichy, Madurai, Coimbatore and Tirunelveli. The findings of the survey showed an overall sero prevalence of 23%, as of the 22,904 samples tested, 5,316 participants had antibodies present. Further, the highest sero positivity was observed in Thiruvallur district at 49%, while the lowest was observed in Nagapattinam district at 9%.

The press note added that the first phase of the survey held in October-November 2020 by DPH&PM recorded a sero positivity of 31% as 6,995 people out of the 22,690 blood samples that were taken, were found to have antibodies. The DPH&PM pointed to the timing of the surveys, waning of antibodies, vaccination coverage and emergence of SARS CoV-2 variants to be possible reasons for decline.

The DPH&PM said, “The first phase of the survey was held in August 2020, nearly four weeks following a high surge in COVID-19 cases. This possibly gave sufficient time to develop antibodies in the community. However the second phase survey was held at the beginning of the second wave.”

“Vaccination was prioritised for those above 45 years till March this year, which left other groups vulnerable to the virus. Meanwhile, waning of antibodies in the five-month gap between first and second survey (November 2020 to April 2021) could be one of the reasons for low sero prevalence,” the press note stated.

Subsequently, no cases of SARS CoV-2 variant were reported in the first survey, whereas several variants were detected in the community including the ‘Delta’ strain that is said to have originated in India. Reports suggest that a third survey is planned in July-August 2021, considering the current decline of COVID-19 cases along with intensified vaccination drive for all ages in Tamil Nadu.

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