Sero survey: Delhi past 50%, near herd immunity

THE FIFTH round of serological surveillance conducted in Delhi has suggested that more than 50 per cent of those surveyed have developed antibodies against Covid, officials told The Indian Express.

This is the highest seroprevalence found during surveys conducted by the Delhi government since the Covid outbreak and, according to experts, indicates that the city is moving closer to achieving herd immunity.

“In one district, the prevalence is around 58 per cent, which clearly shows that a large number of people have developed antibodies against the virus,” said a senior government official. Authorities had collected 28,840 samples — the most during such an exercise in Delhi so far — as part of the survey that ended Friday.

Only Pune has recorded prevalence of more than 50 per cent in some areas, but that was several months ago. In most other cities, seroprevalence studies have shown antibodies in about 30 and 40 per cent of respondents.

Given that detection of new cases has been declining over the last four months, some experts and scientists believe that a much higher proportion of the population might have already got infected. The results of the Delhi serosurvey lends evidence to this.

In August last year, the first such survey conducted by the Delhi government showed 29.1 per cent had antibodies. The figure dropped to 25.1 per cent in September and stood at 25.5 per cent in October. The National Centre for Disease Control (NCDC) had also conducted a serosurvey in July, where antibodies had been detected in 23 per cent of those surveyed.

The latest survey kicked off in Delhi on January 11 and samples were sent to the Institute of Liver and Biliary Sciences (ILBS) for testing, with an initial report being submitted to the office of Delhi Health Minister Satyendar Jain.

Dr Shobha Broor, former head of microbiology at AIIMS, said that the survey “shows that there have been many asymptomatic infections in the city and we are approaching herd immunity”.

“The number may go up to 70 per cent in the next round as more people would have got vaccinated by that time. Having presence of antibodies will not affect the ongoing vaccination drive in any case. Those who have developed antibodies can go ahead and take the shot as the vaccine will further boost the response,” she said.

Dr Neeraj Nischal, associate professor at the department of medicine, AIIMS, said the high prevalence of antibodies may help in further controlling the pandemic.

“It shows that a lot of asymptomatic infections did happen in the course of the pandemic, which were not reported at that time. This is our natural way of acquiring herd immunity as approximately 50 per cent have developed antibodies against the virus. This coupled with the ongoing vaccination drive will help in achieving herd immunity, and controlling the pandemic will be faster than expected,” he said.

A serological survey is meant to check prevalence of a disease in a population, by detecting the presence of specific antibodies against the virus. The serological test is performed to diagnose infections and autoimmune illnesses. It can also be conducted to check if a person has developed immunity to certain diseases.