The survey, carried out by Thyrocare across the country, involved 9,041 people in Kolkata and 22,589 people in Bengal. “This is a blood-based serology test, similar to those done to detect other viruses like HIV, HBV or HCV. The test finds out whether a person has got infected ever for that virus or not. In case of Covid, too, it tells us if the person has got exposed to the virus in the days, weeks or months leading to the test,” said A Velumani, managing director of Thyrocare.
“It would be great if the government comes up with its own survey to find out the prevalence of sero-positivity on the lines of the one done in June in a few wards of KMC. This, along with the positive cases will help health officials to know how close we are towards achieving herd immunity,” said a doctor attached with SSKM Hospital.
According to Belle Vue Clinic internal medicine consultant Rahul Jain, the data is encouraging since it indicates that a substantial section has been exposed and developed immunity. “Apart from proving that Covid cases are mostly mild, it also reveals that the mortality rate is abysmally low. More importantly, it suggests that we are on the way to developing a herd immunity which will be achieved once 50%-60% of the population turns sero-positive,” said Jain.
He explained that once more people develop immunity and successfully stave off the infection, they will block transmission, save the vulnerable from the virus and eventually lead to its end.
“Once half the population develops antibodies, there will still be a vulnerable core with lower immunity and co-morbidities who will remain susceptible to the virus but will have a far lower risk of infection due to the shield provided by those with antibodies,” added Jain.
In Kolkata, the survey covered corporate employees, housing society residents, passers-by and was carried out over a period of five-six weeks.
“We did not start this with any aim. Initially, corporates wanted their employees to be tested for knowledge and we gradually started testing others and the survey took off,” said Velumani. He, however, warned that sero-positivity didn’t necessarily ensure that the virus can’t strike again.
In the rest of the state, Darjeeling had the highest positivity rate at 41% while Bankura had the lowest with just 6.3%. Neighbouring Howrah surpassed Kolkata with a rate of 29.4%.
Within the city, the Posta area (Kolkata 7) recorded the highest positivity rate with 47% while Kolkata 9 was the lowest at 14%.
“Since the national antibody index is 27%, around 30% of the population is already exposed at the cost of 80,000 deaths. This also means that 2,40,000 people may die by the time 90% gets exposed, which is likely by end of March, 2021. If a vaccine does not reach by the end of 2020, it may not be of any use subsequently,” said Velumani.