Prior to the second wave of the pandemic, around 23% of Tamil Nadu’s population, excluding Chennai’s, may have been exposed to SARS-CoV-2, the virus that causes COVID-19, according to the second sero-survey done by the Directorate of Public Health and Preventive Medicine during March-April.
There was an overall reduction in the average antibody prevalence, compared with the first survey conducted during October-November 2020, when the State’s sero-prevalence was 31.6%.
A total of 22,817 blood samples were collected from 45 health unit districts (HUDs), other than Chennai, for the second survey. Of the 22,721 samples processed, 5,242 returned positive, indicating a sero-prevalence of 23.1%. While the Poonamallee HUD recorded the highest sero-prevalence of 50.6%, Nagapattinam recorded the lowest of 8.9%.
Three HUDs in the northern region — Tiruvallur, Chengalpattu and Kancheepuram, which were among the hotspots in the first wave of the pandemic — recorded 48.3%, 42.5% and 37.7% respectively.
With the results pending for Chennai alone, the city’s sero-positivity was 18.4% in the first sero-survey, followed by 30% in the second survey done by the Indian Council of Medical Research.
The current hotspots — Coimbatore, Tiruppur and Erode — have shown an overall antibody prevalence of 20.5%, 23% and 17.2% respectively in the second sero-survey. This, the Directorate noted, indicated “low antibody prevalence”. It showed that neither natural antibody nor vaccine-induced antibody was widely prevalent in these hotspots.
So the challenges remain, say health officials. The lesson learned is to push vaccination more to improve antibodies and reduce the vulnerability.
”The survey was done before the second wave. It indicates that if we look at vaccination levels of about 13% and sero-prevalence of 23%, the State has almost 64% population that can be termed vulnerable. So we have to keep following non-medical methods and focus on rural and other districts with low sero-prevalence. Also, vaccination of the vulnerable would help,” Health Secretary J. Radhakrishnan said.
Genome sequencing
The State had sent 518 samples for Whole Genome Sequencing to InStem, Bengaluru. The results of 192 samples released as of now have indicated SARS-CoV-2 variants in Tamil Nadu, the highest being B.1.617.2, now named Delta by the World Health Organization. About 77% of the samples — 148 — returned positive for Delta.
The sero-survey results should always be interpreted with caution because of a number of factors, said T.S. Selvavinayagam, Director of Public Health and Preventive Medicine. “These include the protective effect — one may or may not get infected, the virulence and infectivity of the virus and the host effect that is the individual’s reactivity to the virus. The variants may or may not have the protective effect. The overall protective effect may be due to the combination of vaccination and sero-positivity due to infection,” he added.