Delhi’s 7th sero survey from next week, will also account for vaccination status

The seventh round of sero-surveillance to estimate the population level exposure to the SARS-CoV- virus that causes coronavirus disease (Covid-19) will begin in the capital next week state government officials familiar with the matter said.

“The survey will begin in another week or even sooner; the files for approval have already been sent to the minister’s office. The survey was initially supposed to happen between August 1 and 10 but couldn’t be done,” said a senior official from the health department who asked not to be named.

Sero-surveillance 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 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.

A senior official from Shahadara district said, “Our teams are ready; the survey is likely to begin at a short notice.” An east district official confirmed, “There were a couple of meeting at the beginning of the month, but the survey did not start. However, our teams are already prepared and we can start collecting samples at a moment’s notice.”

There are likely to be several new parameters included in the survey, according to researchers at the Maulana Azad Medical College who are leading the effort. With a significant proportion of the population having been vaccinated, details of vaccination will be included, which was to be done even during the April survey that was truncated because of the sudden increase in cases.

Till Tuesday evening, 3.57 million people in Delhi were fully vaccinated and another 5.47 million partly vaccinated. The number of people eligible for vaccinations in Delhi is 15 million, which means at least 60% of them have thus far received at least one dose of the vaccine.

Delhi’s high sero-surveillance reading in the last round is understandable; the city was ravaged in April and May by the second wave of the coronavirus disease pandemic with cases rising to 28,395 on April 20 at the peak. The Union territory has seen over 1.43 million cases and 25,080 deaths so far according to the HT dashboard.

The sample collection will be carried out as per the previous methodology to ensure that the surveys are comparable, according to the researchers at Maulana Azad college.

“(ICMR) has advised states to conduct the sero-prevalence studies in their own States/UTs in consultation with ICMR, so that such studies follow a standardized protocol,” the Union government told states in July after releasing the findings of the fourth round of national sero-surveillance, in which Delhi was not represented.

A WHO study done by the All India Institute of Medical Sciences found the population level sero-prevalence of 67%, with samples being collected between March and May. The prevalence is likely to be higher as the antibodies are detected 10 to 14 days after the infection and the peak infections happened in April-end and May-beginning in Delhi.

Another study of around 1,000 people across CSIR campuses in Delhi found the sero-prevalence to be 87%, with samples being collected between May end and early-July.

According to INSACOG (Indian SARS-CoV-2 Consortium on Genomics), many of these people were infected with the delta variant of the virus.