COVID-19: What serosurveys suggest and what they hide

We know the results of serosurveys in five Indian cities now, the most recent being that of Pune, where more than half the sample size surveyed had antibodies. But this doesn't necessarily mean herd immunity. What it does mean is that we need to look much more critically at our numbers of infection and the death toll

By Banjot Kaur
Published: Tuesday 18 August 2020

The results of five serosurveys for the prevalence of novel coronavirus (SARS-CoV-2) infections conducted in several parts of India — with the latest one in Maharashtra’s Pune — have been declared by now. A serosurvey tests the presence of antibodies that are developed after an infection.

The results for the surveys varied because of the time they were conducted at and their respective sample sizes into account. The common interpretation drawn from all them, however, was whether the cities they were conducted in were moving towards herd immunity or not.

The survey in Pune — conducted in five wards of the Pune Municipal Corporation where the incidence of the infection was high among 1,520 individuals — was conducted from July 20, 2020 through August 5, 2020, according to a technical report.

The seroprevalence was found to be 51.5 per cent. This means 51.5 per cent people living in the five wards — Yerwada, Lohiyanagar-Kasewadi, Rashapeth-Ravivarpeth, Kashbapeth-Somwarpeth and Navipeth-Parvati — had Immunoglobulin (IgG) antibodies.

“We just looked into the antibody prevalence,” Ashok Ghose, associate professor of Pune-based Indian Institute of Science Education and Research, told Down To Earth. “Now, what proportion of these antibodies were neutralising antibodies that actually help fight against the virus is something we really do not know at the moment. For that, more studies are required,” he said.

Mumbai-based Tata Institute of Fundamental Research (TIFR) released the findings of the serosurvey conducted in three wards of Mumbai July 28. The findings of R-North, M-West and F-North, the three wards of the Brihanmumbai Municipal Corporation, suggested the prevalence of antibodies in 57 per cent of the slum population and 16 per cent of the non-slum population.

TIFR also did not know yet what proportion of antibodies were neutralising in nature, Ullas S Kolthur, professor at the institute, told DTE August 18. “We are doing a second round of survey in these areas to find that,” he said.

Seroprevalence in Gujarat’s Ahmedabad, Odisha’s Berhampur and in Delhi was found to be 17 per cent, 31 per cent and 23 per cent respectively. Like the surveys in Mumbai and Pune, these exercises found the presence of IgG antibodies and not actual antibodies that create a protection mechanism against the virus.

Experts, thus, suggest drawing the conclusion that the cities are closer to achieving herd immunity, is jumping the gun, until follow-up studies do not explicitly say so.

The prevalence of infection indicated the scale to which it may have spread in the respective localities of the cities where they were conducted. It is prudent to believe a large number of cases are asymptomatic as they were out of the testing net: The infection spread found through the surveys was not reflected in COVID-19 cases detected through tests.

Uncounted COVID-19 deaths?

What these surveys suggest, however, is much more grave, said experts. At least two people associated with the Pune serosurvey — Sanjay Rath, former professor at the National Institute of Immunology and Jacob John, professor at the Christian Medical College, Vellore — said there may be a possibility of the current testing regime missing out on a large number of positive cases and deaths, since the infection was found on such a large scale. An improvement in the country’s testing regime was needed, said both.

There were studies to suggest that even if neutralising antibodies are developed, they drop after a point of time, said Rath. A long-term immunity is, thus, also not reflective in such surveys.

“A clear message from the surveys is the disease outbreak is widespread and the administration needs to pull up its socks,” said Ghose.

John, however, tended to differ a bit. “This view is a little pessimistic. I will not say that entire population found positive for antibodies was protected, but I will still believe a large chunk of the population was infected,” he said. “Otherwise you would have started seeing a large number of re-infections by now,” he added.

Experts — talking about the high seropositivity found in Pune compared to the ones in Ahmedabad, Berhampur and Delhi — said the timing mattered. The localities — at least the ones where people lived in tenements — in Mumbai and Pune showed a similar seroprevalence.

In Pune, the survey was conducted between July and August, while in Mumbai it was done in the first half of July. In contrast, the serosurvey in Delhi was conducted between June and July.

So the later the survey was conducted, the more time the virus had to infect people. One, thus, has to be cautious when comparing how high or low seroprevalence is in respective areas.

Pune slums vs Mumbai slums

The seroprevalence found in people living in hutments and tenements in Pune was 62 per cent and 56.2 per cent respectively, while that of people in apartments and bungalows was 33.2 and 43.9 per cent respectively.

In Mumbai, seroprevalence of slum population was at 67 per cent compared to 16 per cent in non-slum population. Will this be a trend now? “It might very well be simply because people living in cramped settings will obviously find it difficult to maintain physical distance,” Rath said.

What does this say about slums? Since taking recourse to herd immunity may by and large be fraught with complacency, experts suggest a second surge can lead to a high case load in these areas, something avoided in the first wave.

“Let’s say if they did not produce enough antibodies during the first course of infection and they get affected a second time, the current serology of the body at the time will dictate how bad or good the response is going to be,” Ghose said.

Elderly and women

Pune’s survey revealed an interesting figure: Among all age-groups, the lowest infection spread was found to be among those who were 66 years and above. “It is wrong to believe the elderly were more susceptible to catching the infection. They are more susceptible to getting a severe illness once they get infected. These are two different things,” Rath said.

As far as seroprevalence in women is concerned, Mumbai's survey indicated it was marginally higher in them, but Pune’s survey said otherwise: The seropositivity was found to be 52.8 per cent in men and 50.1 per cent in women.

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