Delhi’s 7th sero survey to begin Friday, check infection and vaccination status

After a nearly two-month delay, the state government will on Friday begin the seventh serological survey to estimate the population-level prevalence of antibodies against the Sars-CoV-2 virus (which causes Covid-19) in the Capital, said officials familiar with the matter.

The sample collectors will also record Covid-19 infection and vaccination history. Those not vaccinated will be asked why they have not got the shot yet, as part of the survey.

“We need to understand how many people have been exposed to the virus, how many have received one dose, how many have received both the doses, and the antibody levels in them. We also need to understand why some have not received the shot. A detailed questionnaire on the history will be filled up while samples are collected. We urge those who are randomly selected for the survey to agree to share their blood samples,” the Delhi health department official said, asking not to be named.

Sero surveys are blood tests that check for antibodies to the virus. They are a good measure of the exposure of a population to the virus. They are also a proxy for the level of broad immunity that exists in the population, although the best immunity is that achieved through vaccination.

The survey, which was initially to take place over the first two weeks of August, was delayed. It was then scheduled for the last week of August, was pushed back yet again. It was not immediately clear why the survey was postponed twice, and state health officials declined to comment on the matter.

All data will be captured digitally during the survey, making compilation and data analysis easier, said officials.

“Our team of phlebotomists will collect the samples, and an external team accompanying them will collect the digital data,” said a senior district-level official.

HT previously reported that the Delhi government had a proposal in the pipeline to hire an agency to conduct the survey.

The samples will be processed at laboratories in Delhi government hospitals.

HT on Wednesday reported that a proportion of the blood samples will also be analysed for the levels of neutralising antibodies that specifically target the Sars-CoV-2 virus. “This will also help us in co-relating the level of antibodies with the IgG levels found in the semi-quantitative test, thereby giving us an understanding of how much of the population is actually protected,” one of the researchers said previously.

The team is expecting that with the severe surge in infections in April-May and the increased pace of vaccination, 80 to 90% of Delhi residents are likely to have antibodies against the virus.

The last round of the survey conducted in Delhi in April, the sixth such, had to be stopped midway due to rising cases but 13,000 of the planned 28,000 samples that had already been collected returned a reading of nearly 56%, which means 56 out of every 100 residents of the Union territory were exposed to the virus. The previous five surveys returned readings of 56.13%, 25.5%, 25.1%, 29.1%, and 22.6% respectively.