With Delhi's population of more than 20 million, the findings published Tuesday by the federal health ministry suggest that 4.7 million people have had the virus, almost 40 times the official tally of 125,000.
While praising government restrictions that have limited the spread of the virus, the ministry said the study "indicates that a large number of infected persons remain asymptomatic".
The survey was conducted between 27 June and 10 July.
And more than 75 per cent are still vulnerable, including those at higher risk, Sujeet Kumar Singh, head of the National Centre for Disease Control, told a news conference.
On Wednesday, India's health ministry reported a total of 1.19 million coronavirus cases so far with almost 29,000 deaths.
Maharashtra, Delhi and Tamil Nadu states are the worst affected.
Prime Minister Narendra Modi's government imposed one of the world's strictest lockdowns in late March, but it has been steadily eased in recent months to lessen the devastating economic impact.
But, independently from the federal government, states have been tightening restrictions as case numbers have soared - including in Bangalore, Bihar, West Bengal and parts of Kerala and Tamil Nadu.
Daily new cases have been steadily falling in recent weeks, with fewer than 1,000 new infections recorded on Tuesday - the first time in 49 days - down from a peak of almost 4,000 in late June.
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