As per the results of the survey done in April 2021 by the Tamil Nadu government the overall seroprevalence was 23 per cent.
According to the survey report, a total of 765 clusters were included covering 22,904 samples.
The blood samples were drawn randomly from a village in a rural area or a street in the case of an urban area, not including Chennai.
The sero survey was successfully completed under the supervision of Dr.T.S.Selvavinayagam, Director of Public Health & Preventive Medicine (DPH&PM) and his team at the State and District level.
According to the report, blood samples were collected from a total of 22,904 participants and tested for the presence of SARS CoV-2 IgG antibodies using Chemiluminescence based Immunoassay (CLIA).
All the samples were tested at six referral Labs of DPH&PM located in Chennai, Trichy, Madurai, Coimbatore, Tirunelveli and Salem for the detection of IgG antibodies.
The key findings are summarized below:
-Among the 22,904 samples tested, 5316 individuals had IgG antibodies against SARS-CoV-2 Virus.
-Overall Seroprevalence was 23 per cent.
-Highest Seropositivity was observed in Tiruvallur District (49 per cent)
-Lowest Seropositivity was reported in Nagapattinam District (9 per cent).
The first survey done during October/November 2020 showed a seropositivity of 31 per cent when 22,690 samples were tested and 6,995 were found to be reactive for SARS CoV-2 IgG antibodies.
The survey report said, the sharp decline in Seropositivity to 23 per cent in the second survey from the earlier 31 per cent could be attributed to the following possible reasons:
-The first Serosurvey was initiated around four weeks following the peak cases reported in Aug 2020 during the first wave of COVID-19 in Tamil Nadu. This time gap of four weeks would have given the sufficient time to develop IgG antibodies in the community.
-Unlike the first one, Serosurvey II was conducted close to the beginning of the second wave of COVID-19 in Tamil Nadu.
-Wanning of IgG antibodies during the time gap of five months between first and second survey (Nov 2020 - March 2021) could be one of the valid reasons for low Seroprevalence as reported by several studies published in International Journals.
- Vaccination till March 2021 was prioritized to target a population of Aover 45 years which left other age groups still unvaccinated and vulnerable.
-There were no reported cases of SARS CoV-2 variants in Tamil Nadu during the first Serosurvey whereas several variants were subsequently detected in the community including B.1.617.2 (Delta) strain of Indian Origin.
The third Serosurvey is planned to be carried out in July/Aug 2021 considering the current declining trend of COVID-19 cases coupled with intensified vaccination drive being carried out among those aged over 18 years in Tamil Nadu.