Report of 6th sero study may be filed on Monday; sample size reduced after surge

A report on the truncated sixth round of serosurvey carried out in the Capital in April is likely to be submitted to the Delhi government on Monday, senior officials in the health department said on Thursday.

Nearly 10,000 samples collected across the city over a fivesix day period are being processed by researchers from the Maulana Azad Medical College.

The survey was curtailed after Covid-19 cases started soaring in the Capital, as health care workers balked under the pressure of the fourth wave of infections.

The survey, for which 28,000 blood samples were to be collected from across Delhi, started on April 12, a day after the city reported over 10,000 new cases of the viral infection. Over the next two weeks, Covid-19 cases in Delhi peaked at over 28,000 cases a day.

“The survey had to be stopped midway because cases started rising and a lockdown was implemented. Now, the samples that were collected during the five or six days are being analysed. The researchers are likely to submit their report on Monday,” said an official from Delhi’s health department on condition of anonymity.

The sixth round of the survey was the first one to be conducted after a significant proportion of Delhi residents received a vaccine against Covid-19, hence participants were asked about their vaccination history along with a history of infection, the researchers told HT before they embarked upon the survey.

Delhi has been conducting regular sero-surveys to check the population-level prevalence of antibodies against the Sarscov-2 virus that causes the coronavirus disease. To be sure, the current prevalence of antibodies is likely to be much higher than what will be found in the report of the sixth round since cases in Delhi remained above the 10,000-mark till May 13 — nearly a month after the round began. The state government is likely to conduct another sero-survey soon to check the prevalence of antibodies after this surge.

The previous sero-survey was conducted between January 11 and 21 this year and found that over 56% of Delhi’s population was exposed to the viral infection. However, a brutal fourth wave of infections swept through the city in April-may, despite such high levels of antibodies in January. Experts said this was likely because of the Alpha (B.1.1.7) and Delta (B.1.617.2) variants of the Sarscov-2 in circulation, which are more transmissible.

In fact, a report by Indian scientists from the National Centre for Disease Control (NCDC) and Institute of Genomics and Integrative Biology (IGIB) who carried out genomic analysis of infections found that the Delta variant out-competed the Alpha variant within weeks and went on to spark Delhi’s most devastating wave of Covid-19.

“The new variant is more infectious. Earlier, if two of five in a family were getting the infection, now everyone gets it. That means the r0 (the reproduction number) is higher. Consequently, the herd immunity level will not be 60%, but higher — probably 70 to 85%,” said epidemiologist Dr JP Muliyil.