Delhi serosurvey says novel coronavirus infected 6 mln; how do we read this?

Some six million people in the national capital, out of a total population of 20 million, have been infected with the novel coronavirus (SARS-CoV-2) at some point in time, according to the results of the second Delhi serosurvey. 

The survey was conducted in the first week of August 2020, among 15,000 residents across all 11 districts of Delhi. It was conducted by the Maulana Azad Medical College for the Delhi government. The first serosurvey was conducted by National Centre for Disease Control from June 27-July 20. The seropsitivity at that time had been 23 per cent.

A rise of about five per cent in the serospositivity does indicate that infection made its inroads deeper but a district-wise analysis reveals it was much steeper in a couple of districts. 

How do we interpret this?

While six million people in Delhi had developed antibodies, the flip side of the coin was that a huge chunk of the population was still not exposed to the virus, Delhi’s health minister, Satyendra Jain, said. 

It also meant that people just could not afford to drop their guards yet, Shahid Jameel, a virologist and chief executive of the Wellcome Trust (UK)-DBT India Alliance, said.

“The saving grace is the later one gets infected, the better the chances of coming out of the infection due to several reasons like better awareness and also better health facilities that are prepared to deal with patients,” Jameel said.

“But since the vaccine is not going to be available for common people before the middle of the next year or even later, the infection will spread further in Delhi or anywhere else,” he added.

A number of experts and Jain himself said they were expecting higher seroprevalance this time. “I am also a tad surprised at these results. Though we did have high seroprevalance this time as compared to the last but the pace at which infection made inroads in Delhi was slower than most people might have thought,” Jameel said.

Pune vs Delhi

The Pune Municipal Corporation released the sero results of five sub-wards earlier this week. Those wards showed a seropositivity of 51 per cent.

Since it was late July-early August, the results reflected the time the virus got for transmission, experts said, explaining the high rate. Delhi’s second serosurvey was also conducted early August but the seropositivity was just 28.35 per cent. How can this be contextualised?

“To compare it with Pune, we need granular data. For instance, in the case of Pune, it was made clear that the survey was conducted in high incidence areas. The Delhi government has not said anything on this,” Gagandeep Kang, the former head of Translational Health Science and Technology Institute, said.

“For Pune, we also got to know what the different seropositivity rates were for people living in hutments and tenements on the one hand as well as apartments and bungalows on the other. For Delhi, this data has also not been shared yet,” she added. 

Also, the assays used to conduct the serosurveys were different for Pune and Delhi. On paper at least, the sensitivity of assays used in Pune is better than the ones used in Delhi.

Herd immunity or not? 

Jain did take recourse to the most debatable tool to fight this pandemic during his press conference — herd immunity. He hoped that seropositivity would reach 60 per cent, since that would mean Delhi had won the battle. To understand the sero trends, such surveys would continue to take place, he said.

Jayaprakash Muliyil, one of India’s leading epidemiologists, agreed that 50-60 per cent seroprevalence would be enough for a place like Delhi.

But others, including Jameel, said that serosurveys only suggested the presence of IgG antibodies and not the actual proportion of neturalising antibodies, that actually fought the virus.

Muliyl, on the other hand, believed that IgG antibodies coordinated well with neutralising antibodies and therefore, one might draw some solace from these serosurvey results. But to get any concrete results on this, studies were needed.

“So far, we have had many serosurveys in India. As far as I know, nobody, other than Pune-based researchers are looking into neutralising antibodies as a follow-up exercise,” Jameel said.

While the debate on herd immunity continues, it is also imperative to understand why some districts in Delhi had a much sharper rise than the others to know where the administration is going right and where it is not. “It is time the Indian Council of Medical Research starts doing these studies wherever serosurveys have been done,” an expert said.

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