5th sero survey begins in Delhi, likely to last 10 days

State government officials have started collecting blood samples of over 27,000 people for a fifth round of the serological survey to assess the population’s exposure to the coronavirus disease (Covid-19)

State government officials have started collecting blood samples of over 27,000 people for a fifth round of the serological survey to assess the population’s exposure to the coronavirus disease (Covid-19).

This is the first serological survey being undertaken after the October round, and will likely reflect the surge in infections that took place in November, when Delhi saw its third and worst wave of Covid-19. (Representative Image) (Raj K Raj/HT PHOTO)

This is the first serological survey being undertaken after the October round, and will likely reflect the surge in infections that took place in November, when Delhi saw its third and worst wave of Covid-19, when the Capital dealt with a severe spike in new cases and deaths. During that month, Delhi added its highest single-day case spike, adding 8,593 infections on November 11, as well most single-day deaths, logging 131 fatalities on November 18.

The collection of samples began on Monday and is likely to last for 10 days.

“Around 100 samples will be collected from each ward for the current round of the serological survey. This time, sample collection will take longer because of the vaccine roll-out,” said a senior official from Delhi’s health department.

In the October round of the sero survey, the population level prevalence of antibodies against the Sars-Cov-2 virus increased only marginally from the previous round by 0.4%. This meant that there were only 4,000 infections per million population between early-September and mid-October.

In the round before that, the recorded seroprevalence actually dropped four percentage points from 29.1% in August. This fall was likely due to a change in the technique of sample collection.

However, the authors of the sero survey also warned of diminishing antibodies in those who had the infection months ago.

“During the previous round of the survey, we realised that antibody levels were becoming undetectable in people who had the infection months ago. Some studies show that antibodies can last up to 90 days. So, this time around, we will be able to judge the exposure during the November surge,” said one of the researchers involved in the project, who asked not to be named.

In June-end , 22.6% of the 21,000 people sampled had antibodies. This shot to 29.1% among the 15,000 people sampled in August, and then dropped to 25.1% among the 17,000 people sampled in September. In October, it had gone up to 25.5% with a sample size of 15,000.