Around 15% of Delhi’s population appears to have been infected with the coronavirus disease (Covid-19), with the rate much higher in some pockets, according to initial results of antibody testing of 22,823 people across the Capital.

The overall trend of a Delhi-specific sero surveillance, conducted by the National Centre for Disease Control (NCDC) and Delhi government across 11 districts between June 27 and July 5, suggests that certain pockets have developed widespread transmission but are still amenable to containment, according to government officials familiar with the findings of the report.

The interim findings of the report are higher than the national average of positivity rate for tests — around 10%.

To be sure, health experts have said that many people around the world may have had Covid-19 (and since recovered) without realising it because they suffered few or very mild symptoms. A similar survey in New York, for instance, suggested that nearly 14% of the population there had been infected by the virus and recovered.

“Interim data indicates the average cross-sectional prevalence looks to be around 15%, which has even gone up to 25% in a few zones. The entire data is not out yet so there may be some variation in the final result, but the overall the trend suggests certain pockets have developed widespread transmission that is still amenable to containment,” the official cited above said, requesting not to be identified.

In a serological surveillance, or sero survey, blood serum of a group of individuals is tested in order to understand the prevalence of a virus in that region.

The final result of pilot sero survey done by the Indian Council of Medical Research (ICMR) across 21 states in April is also being processed, and the research body is preparing to launch a follow-up sero survey soon, officials said.

“These studies are subject to peer review, and currently ICMR’s pilot survey that was done around mid-April is being peer-reviewed. It will be put out in public domain once the process is complete,” said Rajesh Bhushan, officer on special duty, health ministry, during a media briefing on Thursday.

He also announced that a follow-up sero survey by ICMR also in the process of being conducted. “There is another sero prevalence study being planned by ICMR across India as a follow up to their mid-April study. For Delhi’s sero survey, 22,823 samples were collected between June 27 and July 5 across 11 districts by the state government and National Centre for Disease Control. It is being analysed in 15 odd laboratories,” Bhushan said.

The blood samples are tested for detecting IgG antibodies. The IgG antibodies generally start appearing after two weeks of onset of infection, once the individual has recovered after infection and last for several months. Therefore, the IgG test is not useful for detecting acute infection but indicates episode of Sars-CoV2 infection in the past, say experts.

“Merely knowing the volume of people who have been infected will only provide the sense of the spread of the disease; what we must try to know is the quantity and quality of antibodies being produced against the virus. Also, for how long these antibodies stay in human blood so as to know if it’s enough to offer protection against the disease,” says Dr Lalit Kant, former head of epidemiology division, ICMR

“Our mortality rate is low as compared to other countries such as Italy or US. People dying due to a disease depend on several factors, of which most important is medical care. Fortunately for India, our medical infrastructure is not overwhelmed. We can manage our patients well. Elderly are at high risk, so they should strictly follow preventive measures,” says Dr GC Khilnani, former head of pulmonology, AIIMS, Delhi

Union health minister Harsh Vardhan, in a briefing on Thursday about the prevalence of Covid-19 in the country, said that there was no need to panic and that most cases and deaths were reported from limited geographical areas. “We have had a detailed discussion with experts and it is reassuring to know that 90% active cases are restricted to only 8 Indian states, and 80% active cases are in 49 districts. Six states have reported 86% of the total Covid-19 deaths,” Harsh Vardhan said. His comment was not linked to the sero survey.

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