A total of 23 per cent of people the state had antibodies present against COVID-19, results of the second phase of the state-wide cross sectional Sero survey, conducted in the month of April 2021, by the Directorate of Public Health and Preventive Medicine revealed on Sunday. Highest positivity of 49 per cent in Tiruvallur and lowest of 9 per cent in Nagapattinam was being observed.

A total of 765 clusters covering 22,904 samples from all the districts, except Chennai was conducted as part of the survey, headed by Director of Public Health and Preventive Medicine Dr TS Selvavinayagam. Of the 22,904 samples tested, 5,316 individuals had IgG antibodies.

As part of the Sero survey, each cluster consisting of 30 participants was drawn randomly from rural area or urban area and 22,904 samples were tested for the presence of SARS COV-2 IgG antibodies using Chemiluminescence based Immuno Assay (CLIA).

The phase one Sero survey revealed a positivity of 31 per cent in Tamil Nadu. The Director of Public Health and Preventive Medicine Dr Selvavinayagam said that the decline in Sero- positivity from first phase is because the first Sero survey was initiated around four weeks after the state reported a surge in cases in August 2020 during the first wave of COVID-19. There was a time gap of about four weeks that led to antibody development. However, the gap between the first and second wave was about five months and the vaccination for the age group of 18-44 was done later than the survey.

The report added that there were no reported variants of SARS CoV-2 variants in Tamil Nadu during the first Sero survey, whereas several variants were subsequently detected in the community including B.1.617.2 (Delta) strain of Indian Origin later.

The state health department is planning to conduct the third Sero survey in July or August this year. It is being considered as the most appropriate time, given the current declining trend of COVID-19 cases and with intensified vaccination drive being carried out among those 18-44 years in Tamil Nadu.