The prevalence of antibodies to the Sars-Cov-2 virus in Delhi fell by more than four percentage points in a month, officials told the Delhi high court citing the recent serosurvey, yielding an epidemiological finding that now skews estimates of how many people may have been exposed to Covid-19 in the national capital.
The presence of antibodies, which develop after a person has had an infection, is only expected to increase within a population with an outbreak.
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But it was 29.1% among the 15,000 people sampled in August, falling to 24.8% among the 17,197 people sampled in September, according to presentations by the Delhi government.
The decline may be due to a change in the sampling method to represent people’s socioeconomic conditions, said the researchers who designed the survey.
“The sampling method we have used this time is more representative; samples were collected based on where people lived: planned colonies, unauthorized colonies, etc. ”Said Pragya Sharma, professor of community medicine at Maulana Azad Medical College (MAMC), who was part of the team that designed the survey, led by the dean of MAMC. Nandini Sharma.
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If this is the correct hypothesis, it means that the level of exposure is one to four individuals, and the 29.1% projected in the previous survey was incorrect due to unbalanced sampling.
Experts said the change in sampling criteria meant that comparing the last two surveys was similar to comparing “apples and oranges.”
“Ideally, seroprevalence should not decrease, especially when a lockdown is taking place and more and more cases are reported. The pattern will become clear after a couple more rounds of serological surveys, ”said Dr Amit Singh, associate professor at the Center for Infectious Diseases at the Indian Institute of Sciences in Bangalore.
The latest survey sampled people based on whether they lived in five types of accommodation: planned colonias, resettlement colonies, urban slums and JJ colonies, urban and rural villages, and unauthorized colonies.
It found that the prevalence of antibodies was lower among those in planned colonies: 22.9% compared to 25.9% in other areas. This could be because it may be easier for people living in planned neighborhoods to maintain social distance and hygiene, according to the survey.
Some researchers added that the other reason for the drop could be a drop in antibodies among people who may have been infected longer.
“The seroprevalence for August was based on infections that had occurred a month ago; Delhi had seen an increase in cases in late June and July. This could be the reason for the high prevalence of antibodies in August and then the dissipation of antibodies resulted in a lower prevalence in September, ”said Dr. Sharma.
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However, studies on immune responses have not conclusively confirmed this trend over the time period in which the three serosurveys were conducted. “There is some data, not very strong, that shows that the antibodies decrease after three or four months, but it would not explain a 4% drop,” Singh said.
Dr Shobha Broor, former director of the department of microbiology at the Indian Institute of Medical Sciences (AIIMS), also said that “there should be no decrease in seroprevalence.”
“We need to look at the minutes of how the samples were collected and how the data was analyzed,” he added.
A spokesperson for the Delhi government said that the sampling for the second survey was conducted at the district level, while for the third sample, 280 divisions were selected: 272 MCD districts and four divisions each from NDMC and the Delhi Cantonment Board. . “Therefore, the third serosurvey was much more representative of the prevalence of antibodies at the population level,” he said.
Officials from Delhi’s health departments previously said, although they asked not to be identified, that the September sampling showed the prevalence of antibodies has risen to about 33%.
The Delhi High Court detained the government for disclosing these findings.
Why is the Delhi government so eager to deliver the survey report to the media and not to the court? Don’t portray the press as unreliable. Don’t mess around with the court like this … Have you issued a clarification? ”Said a bench of Judge Hima Kohli and Judge Subramonium Prasad.
Surveys from Delhi have consistently shown that more women than men have been exposed to Covid-19, which is the opposite of what the survey found nationwide. In the current round of surveys, 30.4% of the participating women had antibodies compared to 27.3% of the men. The highest seroprevalence was recorded among women from Northwest Delhi at 33.1%.
The study authors recommended that rounds of serosurveillance should be performed at longer intervals, currently performed during the first week of each month, depending on resource limitations.
“Future rounds of the sero survey should have a greater focus on ensuring representativeness of the sample in terms of type of settlement and socioeconomic status,” the study said.
The court’s warning came while listening to a guilty plea filed by lawyer Rakesh Malhotra, who had sought to increase evidence in the city.
As of the latest date, the court had asked the Delhi government to present the results of the third serosurvey in court and not release them to the media.
During the hearing, the Delhi government also submitted a status report stating that Geospatial Delhi Limited (GSDL) was requested to map the fixed test sites to ensure easy access to Delhi residents.
The government told the court that the final map has been shared with the Delhi Metro Rail Corporation (DMRC) with the request to display it in prominent places / sites in major metro stations (at entry / exit points and others deemed appropriate) for the information of the general public.
On the issue of providing access to testing through Mohalla’s clinics and community centers, the government told the court that the need for more testing sites is a component of the overall testing paradigm that the Committee of Experts is considering. and that a decision will be made based on what the panel decides.
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