Seventh sero survey likely to begin in the Capital this week

After delays going back to the second wave of the pandemic, the seventh serological survey is likely to begin this week in Delhi, when 28,000 people will be tested for exposure to Sars-CoV-2. 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.

“In a subset of the samples collected, we will check the levels of neutralising antibodies. 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,” said a researcher working on the surveys conducted by the Delhi government.

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 just 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 survey should ideally have been conducted eight weeks after the infection peak we saw in April-end. That was the time we would have seen the highest level of antibodies in those who were exposed to the virus during the second wave,” another researcher involved in the survey said. “However, the good thing is that the neutralising antibodies will be measured in a proportion of the people.” Both the researchers declined to be named.

The seventh survey will be conducted when 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. The survey will include questions about vaccination history, similar to the survey conducted in April, which was aborted due to the surge in infections.

Delhi has been conducting serological surveys since June-July last year. The last complete round, conducted in January, had found the serological prevalence to be 56.13%, almost double the prevalence in the previous survey conducted in October, showing high levels of exposure during the November surge in infections in the city. The result from the April survey showed a similar exposure level detected during the January survey.

“This time, however, we are expecting to find antibodies in 80% to 90% of the population, either through a previous infection or because of vaccination,” said the first researcher quoted in the story. The researchers in the report of the truncated April survey had said that another round of survey must be conducted in June-end or July.

The seventh survey was initially planned to be conducted between August 1 and 10. “The survey will begin in another two days,” the researcher said.

At the end of June 2020, when the first survey was conducted, 22.6% of the 21,000 people sampled had antibodies. This shot up to 29.1% of the 15,000 people sampled in August. It 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.