Phase V of COVID-19 survey finds 97% seropositivity among the elderly in six districts of Tamil Nadu

Phase V of COVID-19 serosurvey in Tamil Nadu shows 97% seropositivity among elderly, indicating high immunity levels.

Phase V of the COVID-19 serosurvey, conducted this April among elderly persons in Chennai and five districts, has found a 97% seropositivity (presence of IgG antibodies to SARS-CoV-2). Of all five phases of serosurveys taken up in Tamil Nadu, the latest has recorded the maximum protective level of COVID-19 antibodies even after a three-year gap as the last serosurvey was done in December 2021.

According to the Directorate of Public Health (DPH) and Preventive Medicine-State Public Health Laboratory (DPH-SPHL), 3,643 blood samples were collected from elderly persons as a part of the Elderly Panel Survey conducted in Chennai, Tiruvallur, Tiruvannamalai, Tiruchi, Dharmapuri, and Kanniyakumari districts in April.

Preliminary analysis of 1,214 samples, which were tested for SARS-CoV-2 IgG, found the protective levels of immunity at 97%. Considering the three-year gap in conducting Phase V of the serosurvey, the prevalence of protective level of SARS-CoV-2 IgG is “very high and significant”, the DPH-SPHL said in a release.

An effective vaccination drive by the Tamil Nadu government may be responsible for the high level of protection against the SARS-CoV-2 virus. In addition, natural infection and subclinical infection due to the weakened circulating Omicron sub-lineage of SARS-CoV-2 could have also enhanced the development of antibodies. Owing to the high seroprevalence of antibodies, the virus could not cause severe illness, while deaths are almost nil in the State, it said.

Health Minister Ma. Subramanian told reporters that this survey had shown that the immunity levels had increased in the State. “In this situation, a non-virulent strain (of SARS-CoV-2) may not cause much impact, but it is important for persons with co-morbidities, pregnant women, immune-compromised persons, and the elderly to take precautions while visiting public places,” he said. While reiterating that there was no need to panic, he said it had been advised that the elderly, immune-compromised persons, persons with co-morbidities, and pregnant women should wear masks, wash their hands frequently, and maintain physical distance at public places.

In the first serosurvey, done in October 2020, the seroprevalence was 32%. This was followed by the second phase in April 2021 that recorded a seropositivity of 29%. Phase III in August 2021 and Phase IV in December 2021 found a seroprevalence of 70% and 87% respectively.

SARS-CoV-2 IgG antibodies were detected from the community samples with the CLIA technology. Each phase of the serosurvey covered more than 25,000 samples selected by population-proportionate sampling across the State. Cluster pouches were used for uniformity in the sample collection process. The DPH-SPHL functioned as the apex laboratory for the State to provide lab training, logistics support, quality assurance, testing and reporting of the COVID-19 serosurvey carried out in the State, the release noted.