Over 23% Delhi residents have COVID-19 antibodies, shows sero-survey data; NCDC says 77% still susceptible
The sero survey was conducted from 27 June to 10 July, during which a total of 21,387 samples were tested by the NCDC in collaboration with the Delhi govt.
New Delhi: Nearly a quarter of people surveyed for sero-prevalence in Delhi have been infected by the coronavirus, the Union Health Ministry said on Tuesday, while a top official said the study also showed the remaining three-fourth population were still vulnerable and called for continuing strict containment measures.
The sero-survey results came on a day the daily cases again surpassed the 1,000. On Monday, Delhi reported less than 1,000 COVID-19 cases for the first time in seven weeks.
The survey showed nearly 23 percent people had developed anti-bodies for the infection, said National Centre for Disease Control Director Dr Sujeet Kumar Singh.
However, the remaining 77 percent are still vulnerable and the containment measures need to continue with the same rigour, Singh told a press briefing.
The survey was done to estimate the proportion of people exposed to coronavirus infection 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.
Delhi has a population of nearly 2 crore. Eight out of Delhi's 11 districts have more than 20 per cent of sero prevalence, Singh said.
A senior Delhi government official said the survey captured the state of recovery from COVID-19 infection in mid-June, which was a month ago.
"We will consult with public health experts and epidemiologists to determine if the future course of Delhi's strategy against COVID-19 should change in light of these survey results or not," the official said.
Delhi recorded 1,349 fresh COVID-19 cases on Tuesday, taking the caseload in the city to over 1.25 lakh, while the death toll mounted to 3,690 after 27 fatalities were recorded, authorities said.
The number of active cases stood at 15,288, marginally up from 15,166 on Monday.
The sero survey was conducted from 27 June to 10 July, during which a total of 21,387 samples were tested by the NCDC in collaboration with the Delhi government.
"The results of the sero-prevalence study show that on an average, across Delhi, the prevalence of IgG antibodies is 23.48 percent. The study also indicates that a large number of infected persons remain asymptomatic," the ministry said in a statement.
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 percent and this is an adjusted figure... Eight of the total 11 districts have more than 20 percent sero-prevalence."
"Nearly six months into the epidemic, only 22.86 percent of the people are affected. Eight of the total 11 districts have more than 20 percent of the sero-prevalence," Singh explained.
South-east district was found to have a prevalence of 22.12 percent, Shahadra 27.61 percent, North-west (23.31 percent), New Delhi 22.87 percent, Central 27.86 percent, South-west 12.95 percent, North-east 27.7 percent, East 23.9 percent, North 25.26 percent, South 18.61 percent and West 19.13 percent.
The number of people selected from each district for the survey is proportionate to the population of that area, Singh said adding people aged less.
"The study represents the picture of Delhi around the third week of June, (June 18 and 19) when the cases were increasing at a high speed as the antibodies take around 14 days to develop," member NITI Aayog Dr VK Paul said.
"The survey findings show that in city like Delhi having high population density, around 23 percent 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, cautioning against letting the guard down as significant proportion of the population is still vulnerable.
Experts, however, said Delhi is still far away from herd immunity.
Shahid Jameel, the chief executive officer at The Welcome Trust/ DBT India Alliance, said if 23.5 percent sero prevalence is seen in Delhi, for a population of supposed 18.7 million, this amounts to 4.4 million infections.
At 3,663 deaths, the infection fatality rate is 0.08 percent, similar to the first ICMR sero prevalence study done in April.
"This tells me three things — infection has spread quite fast, there is no mortality rate reduction since April end and we are still far from herd immunity."
"Delhi has so far done 8.3 lakh tests. If 20 per cent symptomatic of 4.4 million, there would be 8.8 lakh people with symptoms. But if only one positive is picked in 10 tests in Delhi, we are even missing symptomatic people and are definitely under testing even symptomatic," he told PTI.
Anurag Agarwal, director of the Institute of Genomics and Integrative Biology, said the survey suggests about 25 per cent of people have been infected and recovered, after adjusting for sensitivity and specificity.
Since some people may not have made adequate antibodies, the actual number may be as high as 30 per cent. This suggests that interaction pairs capable of disease transmission amongst Delhi residents will be fewer and the transmission chains will be shorter, he said.
Generally, when serology positivity hits 20 per cent, pretty much globally, that is when the first peak happens, Agarwal said.
Delhi and Mumbai seem to have got to the point. Delhi peaked in the end of June. This is not herd immunity and there may still be a second peak or a slowing of the decline when we reopen further, Agarwal added.
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