Tamil Nadu’s population has shown a high COVID-19 seropositivity rate in a recent survey conducted in July 2021 across the state.According to the results, a total of 66.2% of Tamil Nadu’s population has developed antibodies to the SARS-COV-2 virus. Held by the Directorate of Public Health and Preventive Medicine (DPH), Chennai, the recent serosurvey which is the third in a series of surveys done by the DPH, looked for antibodies against the SARS-COV-2 virus in a sample set of 26,610 individuals. By doing this, they were able to identify how much of the population had been exposed to the virus either through natural infection or vaccination.
Out of the total samples, including samples from 888 clusters tested in six labs in Tamil Nadu, SARS-COV-2 antibodies were detected in 17,624 individuals in the survey i.e 66.2%, according to the survey. This is significantly higher than the seropositivity rate detected in the previous two DPH surveys–31% in October-November 2020 and 29% in April 2021. The results also showed that among Tamil Nadu’s districts, Virudhnagar had the highest seropositivity rate of 84% while Erode had the lowest at 37%. Meanwhile, Chennai recorded an 82% seropositivity rate.
The high seropositivity rate in the third survey could be due to two reasons:
>Timing of the survey i.e towards the end of the second wave
> The vaccination drive picking up in Tamil Nadu
To better understand what a high seropositive percentage means, TNM spoke to two epidemiologists who explained that it is a good sign and could minimise the chances of a third wave in Tamil Nadu.
“There are two ways antibodies can go up–natural infection or immunisation. The important thing to remember is that the higher the immunity levels (be it after natural infection or after immunisation), the lesser the chances of a wave among a population,” says Dr Jayaprakash Muliyil, a renowned epidemiologist studying COVID-19 told TNM. This, he says, is because a wave is caused by an accumulated number of infections. “It (wave) is a function of the number of vulnerable people available. In this regard, what the survey results say is that Tamil Nadu is reasonably secure as 66.2 % of the state population has immunity against SARS-COV-2,” he adds.
However, this does not mean that the population will never get infected. Reinfections and breakthrough infections (when fully vaccinated people get infected) can happen. However, chances of these infections turning into serious diseases (pneumonia) or fatal is highly unlikely.
“Breakthrough infections are more common than reinfections. But in both cases, the chance of mortality or serious disease is very minimal. As in you may be COVID-19 positive even after having recovered from the infection for the first time, or having been fully vaccinated. But the severity of the infection will be very less. In Tamil Nadu, a larger chunk of the population developed immunity after getting COVID-19 as opposed to being vaccinated,” Dr Jayaprakash adds, explaining that vaccine shortage slowed down the vaccination drives.
According to available data, only 41.64 lakh people of a 5.8 crore adult (eligible for vaccination) population in Tamil Nadu have been fully vaccinated. “This means that more people have possibly developed immunity after getting COVID-19, as even in Chennai and other cities, vaccination drives have mainly covered elderly people and those above 45,” he adds.
However, the high seroprevalence levels is not an excuse for people to throw caution to the wind, says former Tamil Nadu Director of Public Health Dr Kolandasamy. “A 66.2 % seropositivity means that 33.8% of the population is vulnerable. In percentages this may look very small, but if you look at absolute numbers, these are 2-3 crore people in TN still vulnerable to COVID-19. This is why percentages don’t work with huge populations,” he explains.
Dr Kolandasamy adds that the susceptible population also includes children and teens who have not been included in the COVID-19 vaccine drive until now. “Until immunisation has been extended to children, people should be extra careful and exercise COVID appropriate behaviour including masking and social distancing,” he says, adding that there is also the urgent need to ramp up vaccination drives.