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Delhi's sero-prevalence study has found that around 23 per cent of the people tested had an exposure to the novel coronavirus in the city, which has several pockets of dense population, the Union health ministry said on Tuesday.
Director of the National Centre for Disease Control (NCDC) Dr Sujeet Kumar Singh said the remaining 77 per cent are still vulnerable to the viral disease and containment measures need to continue with the same rigour.
The study was conducted from June 27 to July 10 by the NCDC in collaboration with the Delhi government and involved testing 21,387 samples.
It showed that 20 per cent population in eight of the 11 districts in the national capital have developed antibodies against COVID-19.
The survey also indicated that a large number of infected people remain asymptomatic.
A sero-survey involves testing of blood serum of individuals to check for the prevalence of antibodies against infection.
NCDC Director Singh said, "When the prevalence was adjusted against the sensitivity and specificity of the ELISA test kits, the prevalence in Delhi was found to be 22.86 per cent and this is an adjusted figure... Eight of the total 11 districts have more than 20 per cent sero-prevalence."
He attributed this to the government's efforts to prevent the spread of infection, such as lockdown, containment and surveillance measures, including contact tracing and tracking, as well as citizen's compliance to COVID-19 appropriate behaviours.
Earlier during the day, the ministry had said in a statement, 'The results of the sero-prevalence study show that on an average, across Delhi, the prevalence of IgG antibodies is 23.48 per cent.
'The study also indicates that a large number of infected persons remain asymptomatic.'
The sero-surveillance was done to estimate the proportion of people exposed to the coronavirus in the total population of Delhi and the number of people selected from each district for the survey is proportionate to the population of that area, Singh said.
Southeast district was found to have sero-prevalence of 22.12 per cent, Shahadara 27.61 per cent, northwest 23.31 per cent, New Delhi 22.87 per cent, central 27.86 per cent, southwest 12.95 per cent, northeast 27.7 per cent, east 23.9 per cent, north 25.26 per cent, south 18.61 per cent and west 19.13 per cent.
"As antibodies take around 14 days to develop, the study represents the picture of Delhi around the third week of June, (18th and 19th day of the month) when the cases were increasing at a high speed," NITI Aayog member V K Paul said.
"The findings of the survey show that in a city like Delhi with high population density, around 23 per cent were exposed to the infection and the credit for that goes to the efforts of the government and the best practices followed by the people," he said but cautioned against letting the guard down as a significant portion of the population is still vulnerable.
The Union health ministry underlined that non-pharmacological interventions such as physical distancing, use of face masks, hand hygiene, cough etiquette and avoidance of crowded places must be followed strictly.
For the survey, blood samples were collected from selected individuals after taking written informed consent and then tested for IgG antibodies and infection using COVID KAVACH ELISA approved by the Indian Council for Medical Research (ICMR).
It was one of the largest sero-prevalence studies conducted in the country using the ELISA testing.
The ministry said that the tests performed was not a diagnostic one but only provides information about past infection due to SARS-CoV-2 in individuals who test positive.
"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," Singh said.
Delhi recorded 1,349 fresh COVID-19 cases on Tuesday, taking the tally in the city to over 1.25 lakh, while the death toll mounted to 3,690, authorities said.