Sero survey finds 23% of Delhi's population infected with Coronavirus

A health worker collects a swab sample for Covid-19, RT-PCR test
A sero survey conducted in Delhi has revealed prevalence of covid antibodies in nearly 23 per cent of the population, with the covid curve for the National Capital beginning to see a declining trend registering less than 1,000 positive cases on Tuesday. 

The sero survey was conducted by the National Centre for Disease Control (NCDC) between June 27 and July 1 across 11 districts of Delhi collecting 21,387 samples. 

While the adjusted prevalence of antibodies is 22.86 per cent, some of the districts have shown a higher spread. For instance, in Central and Shahdara districts, more than 27 per cent of the population was found affected.  The sero survey also indicated that a large number of infected persons remain asymptomatic. 

After seeing a daily rise over close to 3,900 in June, Delhi registered some 954 cases on Tuesday. The Health ministry also said that in the first week of June, when Delhi was conducting an average 9,500 tests per day, the positivity was 37 per cent. In July first week, the tests were increased to over 25,000 and the positivity rate has come down to 9 per cent. 

“This gives us confidence in the strategy we have adopted...(the) disease was accelerating in Delhi but without taking any new restrictions but by bringing the same measures with more rigour we could control our numbers,” said V K Paul, member, Niti Ayog and chairman of the empowered group 1.

Does the declining curve indicate that Delhi is past its peak? “We cannot say that yet because there is still a lot of population which is susceptible to the virus...the remaining 77 per cent is still vulnerable and containment measures need to continue with the same rigour,” said S K Singh, director, NCDC. 

Singh also said that an earlier sero survey conducted across the country in containment zones showed sero prevalence of 34 per cent in Ahmedabad and 9-11 per cent in Delhi. 

There are two types of covid antibodies, one of which appears in the body 14 days after infection and the other develops within the fourth and fifth day of catching the virus, and depletes after two weeks. The former has better longevity. However, how long these antibodies last in a person is still being studied.  

“The Delhi sero survey does not give us the picture of present position but shows us what the situation was around 19-20 June,” Singh said. 

Symptoms based screening and testing policy may not be able to assess the burden of infection in the community effectively, hence, periodic sero-surveys are required to assess the extent of infection in the community, he said. 

The health ministry also said that between June 18 and July 21, over 3,63,000 rapid antigen tests were conducted in Delhi and 6.33 per cent tested positive. Of those that tested negative, 2,294 were symptomatic and underwent the RT PCR test. Around 15 per cent of such individuals were found coronavirus positive.   

India is also planning to increase its testing capacity to around 550.000 in the next two weeks and in the medium term it will achieve the target of one million tests per day. 

At present, 180 tests per day per million are being conducted in the country. India’s positivity rate currently is around 8 per cent and the target of the government is to increase the testing so as to bring down the positivity to 5 per cent or lower eventually.

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