The prevalence of Covid-19 infection is several times higher than detected, according to the results of a sero-surveillance survey released by the government on Tuesday.

The survey carried out by the National Centre for Disease Control and Delhi government from the last week of June to the first week of July showed that almost one in four persons living in Delhi has antibodies against the virus. Covering 21,387 persons across 11 districts, the survey revealed that 23.48 per cent of the samples had past infections from which the patients recovered.

Taking the Delhi population at about 16.75 million, as per the 2011 census, the study indicated that at least 3.85 million Delhiites had exposure to the deadly virus.

World over, nearly 80 per cent of the people exposed to SARS-CoV2 remain asymptomatic. Even then, the back of the envelope calculations showed that about 7.70 lakh people — nearly six times the total confirmed cases of 1.23 lakh in the capital — had some symptoms of the infection that were not detected.

Similar antibody tests carried out on over 60,000 people across the country over the last 20 years by private diagnostic firm Thyrocare estimated that nearly 15 per cent of the Indian population have already developed antibodies against the virus causing the infection.

The Delhi Survey found that sero-prevalence ranged between 13 and 28 per cent in different districts in the capital.

77% vulnerable to virus

“Nearly six months into the epidemic, only 23 per cent of the people are affected in Delhi, which has several pockets of dense population. This means that 77 per cent of the population, including elderly and those with co-morbidities, are vulnerable to catching the virus; so containment measures have to continue with equal vigour,” said NCDC Director Sujeet Singh. He also said it would be difficult to say whether the antibodies are good enough to prevent reinfection in people and that no scientific study has conclusively found out for how long these antibodies would last in the body.

VK Paul, Member, Health, NITI Aayog, said the survey presents the picture of the nature of infection as observed in the third week of June.

Rajesh Bhushan, Officer on Special Duty, Ministry of Health and Family Welfare emphasised the need to wear the correct mask. “N-95 masks with respirator valves should not be used, as the chances of getting infected or rendering someone else unsafe by wearing valve masks are higher,” he said.

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