Amid rising Covid cases, Delhi begins sixth round of sero-survey today

Amid rising cases of coronavirus disease (Covid-19), Delhi government will begin collecting blood samples for the sixth round of serological surveillance from today to determine the change in the population-level prevalence of antibodies against Sar-CoV-2 between January and April. The last round of surveillance, for which samples were collected between January 11 and 21, found just over 56% of Delhi residents had been exposed to the virus.

The prevalence of antibodies had more than doubled from 25.5% during the October round of survey to January, largely owing to the huge third wave of cases seen in November when 8,593 cases had been reported in a single day at the peak.

The current wave has crossed the previous record, with 10,774 cases being reported in a day on April 11. The current round of sero survey will see just over 100 samples being collected from each of 272 municipal wards in Delhi, totalling 28,000. The researchers from Maulana Azad Medical College will also take history of vaccination from all the participants.

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The current serological survey uses qualitative tests that can tell whether a person has detectable antibodies against Sars-CoV-2 virus that causes Covid-19 or not. It does not, however, detect the levels of the antibodies or the neutralising effect of it.

A recent study by scientists from Institute of Genomics and Integrative Biology found that over 17% of those who had detectable antibodies lost the ability to effectively neutralise the virus at six months. Another 4.6% lost the antibodies all together after six months.

In June-end, when the first sero surveillance was conducted, 22.6% of the 21,000 people sampled had antibodies. This shot to 29.1% of the 15,000 people sampled in August, and then dropped to 25.1% among the 17,000 people sampled in September (due to a change in methodology and dropping antibody levels in the population, the researchers said). In October, it went up to 25.5% in a sample size of 15,000.