Delhi to begin sero surveillance from Monday, 28,000 samples to be collected

The protocol followed for the current round of the survey will be the same as the previous round – just over 100 samples will be collected at random from each of the 272 municipal wards in the city

After a gap of two months, Delhi is gearing up to conduct its sixth round of sero-surveillance from Monday, according to senior officials from the health department.

Medical staff work with samples taken during a serological survey for Covid-19 prevalence detection in New Delhi in October 2020. (HT Archive)

The survey will measure the increase in the number of people exposed to the infection between January and April.

The protocol followed for the current round of the survey will be the same as the previous round – just over 100 samples will be collected at random from each of the 272 municipal wards in the city. The total sample size will be 28,000.

The fifth serological survey in Delhi to detect Covid-19 antibodies showed that 56.13% of the 28,000 people sampled tested positive, the government said in February.

The questionnaire will ask for a history of vaccination and previous infection in people whose blood samples are collected to test for the presence of antibodies against Sars-CoV-2 virus. “The participants for the survey will be selected at random and not depending on whether they have been vaccinated. However, since the vaccination drive started between the two surveys, we will naturally ask the participants for their history of vaccination,” said a senior official from the health department, requesting anonymity.

Without the information about a participant receiving the jab, there is no way of knowing whether the antibodies developed are due to an infection or vaccination. The test kits used for the surveillance are qualitative ones, meaning they can just tell whether a person has the antibodies or not. They cannot measure the level of the antibodies.

“The kits used for sero-surveillance tel us whether a person has antibodies against Sars-CoV-2. Whether through vaccination or through natural infection, the antibodies developed are similar and protect against another infection. Also, if the researchers collect data on the date that a person received the shot, we might be able to guess after what time they develop antibodies,” said Dr Jugal Kishore, head of the department of community medicine at Safdarjung Hospital.

The previous round of sero-surveillance, for which the data was collected between January 11 and 21, found that 56% of Delhi residents had antibodies against Sars-CoV-2, the virus that causes Covid-19. The number was higher still in south-east district where the prevalence of antibodies was 62%. The experts had believed that the high sero-prevalence led to the slowing down of the disease spread in December and January.

However, the infection has picked up pace again with cases surging in the city. Delhi reported 7,437 new cases of the infection on Thursday.

Delhi’s first serological survey was done in June-July and it had shown that 23.4% of people surveyed had developed antibodies against the virus. Similar surveys in August showed that 29.1% of people had antibodies, 25.1% in September, and 25.5% in October.