New Delhi: Around 23% of Delhi’s population could have been infected with coronavirus till the first week of July, a serological survey covering over 20,000 people across the capital’s 11 districts between June 27 and July 10 showed, while also pointing to a large number of infected people being asymptomatic.
The findings of the survey, conducted by the National Centre for Disease Control (NCDC) along with the Delhi government, were released on Tuesday, with NCDC director Dr Sujeet Kumar Singh saying the “adjusted prevalence” of IgG antibodies was 22.86%.
Singh, however, pointed to 77% of Delhiites still being vulnerable and said, “Containment measures need to continue with the same rigour. Physical distancing, use of face mask, hand hygiene and cough etiquette must be followed strictly.”

“Nearly six months into the epidemic, only 22.86 % of the people are affected. This can be attributed to the efforts taken up for preventing spread of infection through lockdown and other containment measures,” Singh said.
“This gives us confidence in the strategy we have adopted. The disease was accelerating in Delhi but without taking any new restrictions… by bringing the same measures with more rigour, we could control our numbers,” Niti Aayog member Dr Vinod Paul, who heads the empowered group on Covid-19 response and the Covid-19 taskforce, said.
An earlier sero survey conducted by the Indian Council of Medical Research (ICMR) in containment zones showed prevalence of 34% in Ahmedabad and 9-11% in Delhi, Singh said. Given that the survey of the wider population happened when cases were high, the prevalence rate gave hope that interventions were working, he added.
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Singh was less definitive than AIIMS director Dr Randeep Gulerai in seeing the drop in infections as indicating that Delhi has passed its peak, saying there was a need to monitor the infection. Paul was more positive and said the current levels pointed to a sharp dip but added out that the virus “likes” people without antibodies and caution must be maintained.
Paul said the data collected through the sero survey was “robust” and would be further used by the government for disease control.
While a part of the population remains uncovered under the survey — given its variables — the government maintained that an attempt had been to cover a representative section of the population by casting the net as far as possible.
“RT-PCR test and rapid antigen tests are helpful in treatment of individual Covid-19 patients, while Antibody Detection Tests help assess the prevalence of infection in the community. It is natural that some people such as asymptomatic people are missed by any surveillance network we put in place. In this scenario, a sero survey helps assess the prevalence of Covid-19 infection in the community,” Dr Singh said.
Of the 11 districts, eight had sero prevalence of more than 20%. Central, north-east and Shahdara districts had sero prevalence of over 27%. After seeing a daily rise of over close to 3,900 cases in June, Delhi registered 954 fresh cases on Tuesday.
The health ministry emphasised that increased testing, leading to early identification, contact tracing and isolation of positive cases had resulted in decline in transmission. In the first week of June, when Delhi was conducting an average 9,500 tests per day, the positivity rate was 37%. In the first week of July, the tests were increased to over 25,000 and the positivity rate came down to 9%. It has now come down further, the government said.
On whether the declining curve of fresh cases in Delhi indicated that it was past its peak, Singh said, “We cannot say that yet because there is still a lot of population which is susceptible to the virus… the remaining 77% is still vulnerable and containment measures need to continue with the same rigour.”
Symptoms-based screening and testing policy may not be able to assess the burden of infection in the community effectively. Hence, periodic sero surveys were required to assess the extent of infection in the community, he said.
Watch COVID -19: ‘22.86% people affected in Delhi according to sero-prevalence study’