Tamil Nadu has an overall seroprevalence of 66.2%, according to the Directorate of Public Health and Preventive Medicine’s phase-III cross-sectional survey conducted during July. Among the 38 districts, Virudhunagar had the highest seropositivity at 84% while Erode had the lowest at 37%.
The State’s seroprevalence — presence of antibodies to SARS-CoV-2 — was 29% during the second survey conducted in April 2021 and 31% during October-November 2020. Of the 26,610 samples tested from 888 clusters for the third round of the serosurvey, 17,624 individuals had IgG antibodies against SARS-CoV-2. The survey covered persons aged 18 years and above.
In fact, the Indian Council of Medical Research’s survey that covered three districts and persons aged six years and above during June-July put the State’s seropositivity at 67.6%.
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Commenting on the State’s overall seroprevalence, Health Secretary J. Radhakrishnan said, “We still believe that all districts should be careful and follow COVID-19 appropriate behaviour and encourage eligible persons to get vaccinated.”
T.S. Selvavinayagam, Director of Public Health and Preventive Medicine, said the results of the serosurvey might look promising but challenges remained. “The risk is that there is still a susceptible population. If Chennai has a seroprevalence of 82% and cases still happen, it means that the virus can still spread if people do not follow COVID-19 appropriate behaviour. The virus is known for variants, and this is a risk. People will be exposed to variants, and this is why we need to follow COVID-19 appropriate behaviour till we reach zero cases,” he said.
He added that serosurveys would be conducted at regular intervals in the State.
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According to the DPH report, the overall seropositivity of 66.2% could be attributed to the timing of the serosurvey and vaccination coverage. The phase III serosurvey was conducted during the declining phase of COVID-19 second wave in the State. Intense vaccination drives conducted from January 16 and inclusion of 18+ age individuals for vaccination would have generated protective antibodies in the population. As of June 10, 97.6 lakh persons got vaccinated — either single or double dose — in the State, it said.
Western districts such as Erode, Coimbatore and Tiruppur have reported around 45% seropositivity which could be attributed to consistent reporting of new COVID-19 cases in these regions, the report added.
Seroprevalence was more than 50% in 33 districts unlike in the previous two surveys in which seropositivity remained under 50%. After Virudhunagar, Chennai had the highest seropositivity at 82% as against 49% in the previous round. As many as 11 districts had seropositivity in the range of 70 to 79%. This included Madurai (79%), Tenkasi (78%), Theni (77%), Tirunelveli (76%) and Ramanathapuram (75%).
In the last two serosurveys, Erode’s seropositivity stood at 17%. After Erode, Coimbatore had the least seroprevalence at 43%. In the second phase, Coimbatore had a seroprevalence of 20%. Tiruppur’s seropositivity stood at 46% as against 23% in the previous survey. Nagapattinam and Namakkal had 47% and 48% seropositivity respectively. While Nagapattinam recorded a seropositivity of nine per cent in the survey-II, Namakkal had 26% seropositivity.
Meanwhile, another serosurvey conducted earlier this month jointly by Greater Chennai Corporation (GCC) and Indian Council of Medical Research’s National Institute for Research in Tuberculosis (NIRT) and National Institute of Epidemiology (NIE) has shown similar results as that of DPH.
Unlike DPH’s study, this study conducted in Chennai included not just the 18 plus population but people aged six years and above. The study, which involved 7,026 samples collected from 51 wards, showed an overall seroprevalence of 78.2 %.
The seroprevalence was 72.2 % in the six to 11 age group, 76 % in the 12 to 17 age group, and 82.5 % in the 18 years and above age group.
Among the 15 Chennai zones, Tondiarpet had the highest seroprevalence of 83.3 % and Madhavaram the lowest at 72.5%. Of the 51 wards, 24 had positivity more than 80%, 21 had positivity between 70% and 80% and six had less than 70%. Otteri had the highest seroprevalence of 94.7 % and Kolathur the lowest of 67.2 %.