Seventh round of sero survey starts in Delhi

The survey, which was initially scheduled to take place over the first two weeks of August, was later postponed to the last week of August, and then again to September last week

Delhi on Friday began collecting samples for its seventh round of serological survey to ascertain the proportion of population that might have immunity against the Sars-CoV-2 virus, which causes Covid-19. The 28,000 samples will be collected by district teams in 10 to 15 days, administration officials said.

A health worker holds blood samples collected for a Covid sero survey in Delhi. (Sanchit Khanna/HT PHOTO)

“The collection of blood samples for the sero survey started on Friday. We have already prepared a schedule for the collection and should finish it within 10 to 15 days,” said a senior official from New Delhi district, asking not to be named.

The data for the current survey is being collected digitally. Another official from the east district said on condition of anonymity, “We faced no problems in using the app; everything went smoothly.”

The survey, which was initially scheduled to take place over the first two weeks of August, was later postponed to the last week of August, and then again to September last week. It is not immediately clear why the survey was postponed twice, with state health officials declining comment on the matter.

Hindustan Times reported on Wednesday that a portion of the samples will be, for the first time, also tested to calculate the levels of neutralising antibodies that directly target the virus.

The last round of the sero survey was conducted in April, just as the second wave of the pandemic swept through the national capital, and had to be stopped midway due to rising cases of Covid-19. But of the 13,000 samples that were already collected before the survey was suspended, nearly 56% returned positive for antibodies. The results were similar to those obtained in the sero survey in January that estimated the exposure after the November surge in cases.

Serological surveys are used by epidemiologists to determine the prevalence of a disease in a population. The testing kits currently in use reveal whether a sample has IgG antibodies against one of the proteins of the virus. They indicate whether the antibody level is high or low, and are called semi-quantitative. The seventh survey will conduct a more expensive test to determine the exact levels of the antibodies in a subset of the samples.

The seventh survey is being conducted at a time nearly three out of four of Delhi’s adult population have received at least one dose of a vaccine against Covid-19, and one in three have been fully vaccinated.