Tamil Nadu’s Covid-19 seropositivity rate stands at 87%

Tamil Nadu has so far administered over 9.42 crore Covid-19 vaccine doses

Eighty-seven per cent of people in Tamil Nadu have been exposed to the Covid-19 virus, the fourth serosurvey conducted between the last week of December 2021 and the first week of January 2022 by the Department of Public Health (DPH) has revealed.

Intense vaccination drive – the state has so far administered over 9.42 crore Covid-19 vaccine doses – is said to be one of the reasons for the higher percentage of seropositivity rate, experts said. The seropositivity rate of 87 per cent is much higher than the 70 per cent recorded in July-August 2021 during the third serosurvey conducted by the DPH.

While the second survey conducted in April 2021 showed just 29 per cent of the population was exposed to Covid-19, the first state-wide survey (October-November 2020) revealed that 32 per cent had antibodies to the Covid-19 virus.

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The DPH conducted the fourth serosurvey between December 27, 2021, and January 3, 2022, before the state witnessed a huge spike in the number of persons testing for Covid-19 driven by the Omicron variant.

Samples were collected from as many as 32,245 individuals covering 1,076 clusters across the state of which 28,071 individuals had a “protective level of antibodies” indicating an overall seropositivity rate of 87 per cent.

“Among the 32,245 participants, 27,324 (85 per cent) were vaccinated among which 24667 (90 per cent) had detectable levels of anti-SARS-CoV-2 IgG Antibodies. Among the 4,921 unvaccinated subjects, 3374 (69 per cent) had anti-SARS-CoV-2 IgG antibodies,” the survey said.

It also concluded that vaccinated individuals have better detectable levels of anti-SARS- CoV-2 IgG Antibodies in relation to the non-vaccinated population. The serosurvey also found that among the 27,324 vaccinated individuals, more than 80 per cent seropositivity was observed in individuals who have taken either Covaxin or Covishield vaccines.

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Tiruvarur in the Cauvery Delta region topped the serosurvey with 93 per cent of the population possessing anti-bodies to Covid-19 followed by Tenkasi (21 per cent), Kanyakumari, Madurai, Perambalur, Sivaganga, and Virudhunagar (all 91 per cent), and Cuddalore and Nagapattinam (both 90 per cent).

While the above-mentioned districts recorded a seropositivity rate of 90 per cent and above, the percentage in all the remaining 29 districts, including Chennai, is over 80 per cent.

“68 per cent of individuals aged between 11 and 18 years, who are unvaccinated at the time of survey also had detectable levels of IgG antibodies against SARS-CoV-2, which is an indirect evidence regarding their exposure to natural infection,” the survey added.

Public health expert Dr K Kolandaswamy told DH that the results of the fourth serosurvey are on the expected lines and they also provide a reason on why the third wave of infections was mild among the vaccinated and those who were infected earlier.

“Another way of looking at the survey is that over 10 per cent of the population is still unvaccinated or not infected by the Covid-19 virus. But the unvaccinated population still runs the risk of getting infected with possible severe disease in the event of a sporadic infection or a fourth wave,” Kolandaswamy, former Director of Public Health (TN), said.

He also said the higher percentage in the serosurvey can be attributed to the robust vaccination drive in the state.

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