Second serological survey in Delhi shows roughly 29% seroprevalence; it was 22.8% in the first study

About 29% of the 15,350 people whose blood samples were randomly collected from across the city for the second serological survey, held earlier this month, have developed antibodies against the deadly COVID-19, according to sources.

‘An increase of 3%-5%’

“The seroprevalence is in the range of 29%, but after making adjustments for sensitivity of ELISA kits used to test the samples, it may come down to the range of 28%. Almost all the 11 districts have shown an increase of 3%-5% compared to the last survey,” a government official told The Hindu.

The earlier survey — done from June 27 to July 4, in which over 22,000 blood samples were collected — had shown that 23.4% of the population had developed antibodies. This was corrected to 22.8%, after adjusting for sensitivity of the kits.

The previous survey was done under the guidance of the National Centre for Disease Control. The second survey was headed by the Delhi government-run Maulana Azad Medical College.

In the latest survey, Central and North districts have shown seroprevalence of about 31%. It is around 18%-19% for South West district, the official added.

After a person develops antibodies against the disease, chances of him/her contracting the virus again are low, said officials.

When a larger part of the population becomes immune to the virus, the chain of transmission of the virus is broken and this reduces the spread of the disease and is called herd immunity. So, a higher value of seroprevalence is favourable.

Results likely this week

The result of the survey is expected to be made public later this week; the final report is still being prepared. This is the second such largescale survey done in the city by the government.

Last month, Delhi Health Minister Satyendar Jain had said that though experts have different opinion on herd immunity, there is consensus that when around “40%-70%” of people get infected and recover (develop antibodies), then herd immunity is attained.

After the last survey, Mr. Jain had said that about a “quarter of Delhi’s total population” had developed antibodies against the virus (extending the survey results to the whole of Delhi). On similar lines, it can now be said that about 29% of Delhi’s population have recovered from the virus and have developed antibodies.

The second survey was done from August 1 to 7, and about 15,400 blood samples were collected from people of different age groups and demographics. Some samples were damaged and the results of around 15,350 samples were used for the study.

The blood samples were then tested in labs run by central and State governments as well as municipal corporations, using antibody testing kits to understand whether the person was infected by the virus and developed antibodies against it.

A positive result would mean that the person has developed antibodies against the virus. So, a seroprevalence of 29% means that 29 out of 100 people tested have developed antibodies against the virus.

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