Chennai: At least 97 per cent of the elderly in Tamil Nadu have immunity against Covid-19, a serosurvey conducted in April 2025 by the directorate of public health revealed.
The fifth edition of the survey targeted elderly persons in districts including Chennai, Tiruvallur, Tiruvannamalai, Trichy, Dharmapuri, and Kanyakumari.
Preliminary analysis of 1,214 samples from this group showed protective immunity of 97%, Health Minister Ma Subramaniam said here on Wednesday.
The state public health laboratory tested more than 25,000 samples per phase, selected through population proportionate sampling across the state, utilising techniques for uniform collection.
“We saw a boost in immunity since 2020. The first phase in Oct 2020 showed 32% immunity, which rose to 29% in April 2021 and 70% in Aug 2021. The same year in Dec, immunity was 87%,” he said. Officials at the directorate attributed the increase in immunity to a combination of factors, including effective vaccination implemented during the pandemic and natural infection, leading to high immunity.
High seroprevalence means that a large percentage of people in the sample population have antibodies in their blood, Director of Public Health Dr T S Selvavinayagam said.
This indicates they were exposed to the SARS CoV-2 virus, which causes Covid-19, and carry proteins, antibodies produced by the immune system in response to an infection or vaccination.
They are a sign that the body encountered the pathogen and mounted an immune response, often leading to some level of protection against future infections, he said.
“We vaccinated more than 95% of the population with at least one dose of the vaccine. Besides this, natural infections due to the circulation of weakened mutations such as Omicron sub-lineages of the virus enhanced antibody development further. This could have contributed to hybrid immunity,” he added.
Officials said high seroprevalence correlates with a positive public health outcome. “We know the virus has been largely unable to cause severe illness. We don’t see Covid-19 causing deaths in people who are infected,” he added.
