Coronavirus | Seroprevalence figure for Tamil Nadu stands at 31.6%

Variations seen across categories; Chennai records 40.9%

A Chennai Corporation health worker shows swab samples collected for COVID-19 testing.

The latest State-level seroprevalence data for Tamil Nadu indicates that 31.6% of its population has contracted COVID-19 at some point in time. Chennai’s figure is higher, at 40.9%.

The results of the survey were recorded in the pre-print journal medRxiv on February 8. Blood samples were drawn from a total of 26,640 adults from across the 37 districts of the State in October-November 2020 for antibody tests.

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As per the seroprevalence data released in October last year by the National Institute of Epidemiology, a unit of the Indian Council of Medical Research, the seroprevalence for Chennai alone was 32.8%. The figure as per the results of the first seroprevalence survey in Chennai, which were released in July 2020, was about 20%. With the total population of the State being 72 million, the present study has indicated that at least 22.7 million people had been infected by November 30, 2020 — the last day of serological sampling. Thus, the actual number of infections is roughly 36 times higher than the number of confirmed cases, which stood at 6,70,392 as of October 15. The State has recorded a total of 8.42 lakh cases and just over 12,000 deaths.

The overall seroprevalence of 31.6% masks substantial variations across the State, as evidenced in the fact sheet. For instance, in Perambalur district, the seroprevalence was 51%, while it was just 11% in the Nilgiris. As is the trend globally, the seroprevalence in urban areas (36.9%) was higher than in rural areas (26.9%). Further, the working age group — between 40 and 49 years for the purposes of the study — had significantly higher seroprevalence than the youth (18-29 years, 30.7%) and the elderly (70+ years, 25.8%). The estimated seroprevalence implies an infection fatality rate of 0.052%, nominally lower than the figures for Karnataka and Mumbai.

“If the percentage of the population actually infected seems lower than we anticipated, we must remember that it might have been the case at the time of conducting the survey. The rate would only continue to go up,” Health Secretary J. Radhakrishnan explained. He added that the State was planning to conduct seroprevalence studies regularly, and the next one would be launched possibly by the end of February.

“This knowledge of population-level immunity is the basis of epidemiology. This is critical for understanding the epidemiology of the SARS-CoV-2 (COVID-19) and formulating effective infection control measures, including the allocation of vaccines,” said T.S. Selvavinayagam, Director of Public Health and co-author of the journal report.

The study was approved by the Directorate of Public Health and Preventive Medicine and the Institutional Ethics Committee of the Madras Medical College.