Over 30% of TN people may have had COVID-19 but experts warn not to drop guard

Perambalur district reported the highest seropositivity at 49.3%, while Chennai stood at 40%.
Coronavirus blood test
Coronavirus blood test
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At least 31.6% of Tamil Nadu’s population may have been exposed to the novel coronavirus, according to a sero-surveillance study carried out in October-November 2020. If the survey is extrapolated to the state’s population (7.21 crore), it suggests that 2.28 crore people may have been infected by the coronavirus in the state. As of February 12, Tamil Nadu reported a total of 8,44,173 coronavirus cases.

The study took blood samples from 26,135 persons across the state and were tested for antibodies against the SARS-CoV-2 virus that causes COVID-19. According to the sero survey study, The Nilgiris district recorded the lowest seropositivity at 12.1%. Perambalur district reported the highest seropositivity of 49.3%. Significantly, Perambalur has recorded a total of 2277 COVID-19 cases until February 12, far behind Chennai, which has reported 2,33,046 infections so far. The seroprevalence in Chennai, however, stood at around 40% based on the study.

Seroprevalence is also reported to be higher among the working population. The sero study conducted in Tamil Nadu is a continuous process and is generally used by the government and experts to understand the pattern of the virus spread. Incidentally, as per the third national sero survey carried out between December 17 and January 8, at least 21.5% of the Indian population may have been exposed to the coronavirus.

Explaining the concept of seroprevalence, Dr Shahid Jameel, Virologist and Director of Trivedi School of Biosciences at Ashoka University told TNM that the overall seroprevalence of 31.6% is the average of the seropositivity recorded across all districts in Tamil Nadu. “30% is too low to have achieved herd immunity. For a virus like coronavirus, herd immunity is estimated to kick in somewhere around 60-70% seroprevalence. This also varies from one virus to the other. Extremely contagious viruses like measles, herd immunity can be achieved only somewhere around 95%,” he explained.

Adding that sero surveys are usually done to understand the spread of the virus, Dr Jameel said that the surveys don’t give the full picture. “It gives an average picture. In my opinion, instead of worrying about whether there would be a second wave or not, it is important to emphasise that even though the numbers have come down, we should continue to follow precautions of mask and distancing. Something similar could happen here, who knows? Especially, the worry is the new variants of the virus that have come in the population now. So, instead of worrying about whether there will be a second wave, we should assume that there will be a second wave and be cautious,” he added.

J Radhakrishnan, Tamil Nadu Health Secretary, emphasised that such reports must be taken with a pinch of salt and not take any single study as a conclusive proof of anything concerning the novel coronavirus. “Sero survey is an important tool to get to know the prevalence of the disease. It is one among the several tools. It is important to interpret it in the right way. The right way to interpret it is that the sero survey gives an indication of the prevalence of the disease. What we should understand is that where seroprevalence is high, the spread of the existing variant may be slow and vice versa. In these places, people have to continue wearing masks since the virus has chances to mutate very frequently,” he said. He added that a higher seroprevalence does not mean that all precautions like wearing masks and frequent handwashing can be abandoned. 

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