The overall Covid-19 seroprevalence in Tamil Nadu is 66.2%, reported The Hindu on Saturday, citing the third serological survey conducted by the state’s health department.

Serological surveys reveal how many people have antibodies against an infection. The percentage of a population that have antibodies against an infectious agent is called seroprevalence, according to the United States Centers for Disease Control and Prevention.

However, it is still not clear how long antibodies last in an infected person and the levels required to prevent reinfection.

The Directorate of Public Health and Preventive Medicine, Chennai, said the survey showed that 17,624 of 26,610 residents tested had antibodies against the coronavirus infection. The highest seroprevalence of 84% was recorded in Virudhunagar district. The survey was conducted in July.

Chennai’s Covid-19 seroprevalence was the second-highest at 82%, followed by Madurai (79%), Tenkasi (78%) and Theni (77%). The seroprevalence was more than 70% in 13 districts.

Meanwhile, the lowest seroprevalence was in Erode district at 37%, followed by Coimbatore at 43%. In five districts, the seroprevalence was below 50%.

The seroprevalence of the state is similar to the results of the latest countrywide serological survey conducted by the Indian Council of Medical Research. Tamil Nadu’s seroprevalence was found to be 66.7% in the ICMR survey, which covered three districts of the state. Madhya Pradesh had the highest Covid-19 seroprevalence among the states at 79%, while Kerala had the lowest (44.4%).

Tamil Nadu on Friday registered 1,947 new Covid-19 cases on Friday, pushing the infection tally in the state to 25,57,611 since the pandemic first broke out in January 2020, reported The Indian Express. With 27 deaths, the toll mounted to 34,050.

The survey

In Tamil Nadu’s first serological survey done in October-November, the seroprevalence was recorded at 31%, according to the healthcare body. It dropped to 29% in the next survey done in April.

The high seroprevalence in the current survey could be attributed to the timing of the survey as well as the Covid-19 vaccination programme, the state healthcare body said.

Tamil Nadu has conducted an “intense vaccination” drive since January 16, when the countrywide inoculation programme began, the healthcare body said. This vaccination drive, coupled with the inclusion of citizens aged 18 years and above, would have generated antibodies in the population, it added.

“As of June 10, 97.6 lakh persons got vaccinated (either single or double dose) in the state,” it said.

The survey was conducted in 888 clusters, in which 26,610 samples were taken. Each cluster had 30 randomly selected participants.

The samples were then sent to six labs of the healthcare body in Chennai, Trichy, Madurai, Coimbatore, Tirunelveli and Salem to check for the presence of antibodies. The survey was conducted under the supervision of Dr TS Selvavinayagam, the director of the healthcare body.