Initial findings of antibody survey in Delhi suggests 33% exposed to Covid-19
The prevalence of antibodies for Covid-19 rose from 29.1% to roughly 33% between August and September, according to the initial findings of the serological survey in Delhi, senior officials in the Capital’s administration said on Wednesday.
The number represents the proportion of people from among 17,000 who were sampled, though the final figures are yet to be officially released. If extrapolated to the roughly 20 million population, the finding implies 6.5 million people may now have been silently exposed to the virus, more than 33 times the 193,526 confirmed infections till September 7 (the date on which the 7-day sampling ended).
“The report has been submitted to the health department and it is currently being reviewed. The final report could have minor variations in the final sero prevalence since data from some wards are also being re-checked,” said a senior government official, asking not to be named.
Covid-19 is known to manifest itself as asymptomatic infections, making its detection hard. Since late July, Delhi has been conducting monthly serological surveys to identify how many people may have had such an infection and silently recovered.
The findings relate to the third survey and, according to the person cited above, the final results are likely to be announced by next week.
The presence of antibodies could prevent a second infection, at least for a short while, though scientists are yet to determine this for a certainty. If protection after an infection lasts for long, a sero prevalence of 60-70% could be enough to lead to herd immunity, when an outbreak begins to die out as the virus finds fewer susceptible people to infect.
In the first survey held between June 27 and July 10, the sero prevalence among those sampled was 22.9%. The following month, this proportion rose to 29.1%. In terms of estimated infections across the city, the July survey projected 4.5 million cases, followed by 5.8 million in August.
This translates to 1.3 million more people being infected in the one-month period in between.
Going by the initial findings, the latest serological survey suggests that fewer people were silently infected in August: roughly 800,000, compared 1.3 million the previous month.
The first survey covered 20,000 people, followed by 15,000 in August and 17,000 in September.
The findings also suggest that Covid-19 testing has been able to catch a larger share of the infections. Comparing sero survey-estimated projections in July and August with the corresponding numbers of confirmed cases shows that the proportion of possible true infections detected rose from 3.3% to 5.5%.
Delhi leads the country in rate of testing per capita.
“A sero survey is necessary to find out the true burden of the infection in the city or state. More importantly it helps identify the number of asymptomatic people who had the disease and recovered from it. Such people, in most cases, are not even aware that they are infected,” said Dr Lalit Kant, former head of epidemiology and infectious disease at Indian Council of Medical Research (ICMR).
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