87% of Tamil Nadu people had antibodies against SARS-CoV-2 before third Covid-19 wave: Serosurvey

CHENNAI: At least 87% of people in Tamil Nadu carried antibodies against SARS-CoV-2, according to the results of the phase -IV serosurvey conducted by the Directorate of Public Health in the last week of December 2021.


Antibodies are protective proteins produced by the immune system in response to the virus. They recognize and latch on to the viruses to remove them from the body. Of the 32,245 people tested for the antibodies against the virus from across the state, 28,071 individuals had a protective level of antibodies – a 17% point increase from 70% in August 2021. This means, at least 87% of the people have a protective cover against that Covid-19 either through vaccination or natural infection.

The study also found that nearly 68% of the survey participants -- aged between 11 and 18 years who were unvaccinated at the time of the survey – had detectable levels of antibodies, director of public health Dr TS Selvavinayagam said.

“These people have acquired antibodies against natural infection,” he said. The highest prevalence was in the 18-44 age group (89.5%) followed by the 45-59 years age group (88.6%) and in people above 60 years, prevalence is 84.5%.

More than anything else, health officials say they now have evidence to show how vaccinations helped. Of the total number of people surveyed, 27,324 (85%) were vaccinated. Nine out of 10 people vaccinated had detectable levels of antibodies, but among the 4,921 people who were unvaccinated only 69% of them carried antibodies.

“Vaccination offers detectable levels of anti-SARSCoV-2 IgG antibodies compared to those who are unvaccinated,” Dr Selvavinyagam said. “We saw high hospitalisations, complications and deaths among people who were unvaccinated, particularly among the elderly and people with comorbid conditions such as uncontrolled diabetes and hypertension,” he said.

The survey, however, does not cover the reinfections as it had been carried out before the Omicron-fuelled third wave, he said. But the study among those who were hospitalised showed that a large number of them were either unvaccinated or had serious complications.

The cross-sectional survey was carried out on individuals aged above 10 years in 1,076 clusters spread across all districts of Tamil Nadu. Each cluster consisted of 30 participants drawn randomly from urban or rural areas. Health workers drew 5 ml of venous blood that was collected in gel tubes. They were sent to three referral labs – the State Public Health laboratories in Chennai, Salem and Coimbatore in a cold chain for testing.

Among districts, nearly 93% of people in Tiruvarur and 92% of people in Tenkasi carried antibodies. Seven other districts -- Perambalur, Madurai, Sivaganga, Kanyakumari, Virudhunagar, Cuddalore, and Nagapattinam -- had antibodies above 90%, while Tirupathur (82%) and Kancheepuram (83%) had the least. While Madurai had a seropositivity of 91%, Chennai reported 88% positivity and Coimbatore reported 85%.