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Explained: What do the Mumbai, Delhi and Pune serosurvey results imply?

After Delhi and Mumbai, serosurvey results from Pune suggest Covid-19 is far more widespread than virus tests show. What are the takeaways about the extent of testing, and the extent of immunity in the community?

Written by Amitabh Sinha | Pune |
Updated: August 23, 2020 10:19:07 am
serological survey, india sero survey results, pune sero survey, mumbai sero survey, delhi sero survey, coronavirus news, indian expressSample being taken for a serological survey in New Delhi this month. (Express Photo: Praveen Khanna)

A serological survey in Pune has provided fresh evidence that the spread of Covid-19 in India has been much larger than what is getting detected through confirmatory tests. In Pune, more than 51% of those who were tested showed the presence of antibodies specific to the novel coronavirus, suggesting that about half the population of this city of 4 million could have already been infected.

At the time the serological survey was carried out between July 20 and August 5, less than one lakh people in the city had been found infected in confirmatory tests. The actual spread could therefore be 20 times larger than the confirmatory tests have detected.

Earlier serological surveys in Delhi and Mumbai have also thrown up similar results. An earlier exercise in Delhi had suggested that the actual spread could be 40 times the number of confirmed cases. A fresh round of survey in the national capital, the results of which are have not yet been officially disclosed, reconfirms that.

India serological survey results: What is the takeaway?

That testing has been woefully adequate, despite a huge increase in testing infrastructure. From a capacity to test just a few hundred samples at the start of the outbreak in March to more than eight lakh tests a day now, there has been a massive upgrade in the testing infrastructure. And yet, the serological tests show that a vast majority of the infected people are still being missed out, particularly those who are not showing symptoms.

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Adequate testing is crucial to containment strategies. It is the only method to identify, and isolate, infected people and their close contacts. The more the tests, the greater the chance of detecting infected people, including those who are asymptomatic. Timely isolation of these can prevent transmission to other people. A higher number of tests, therefore, has a direct bearing on slowing down the spread of the disease.

Considering how widespread the disease is now, going by the results of the seroprevalence surveys, the testing capacity would have to be increased several times to make any meaningful difference to the growth of the disease. That is not something likely to happen overnight. So, while it would still be important to continue testing as many people as possible, it is well past the stage where testing was the most effective tool to flatten the growth curve.

Also read | Hope in numbers: Daily cases stable for two weeks, tests show fewer positives

serological survey, india sero survey results, pune sero survey, mumbai sero survey, delhi sero survey, coronavirus news, indian express People wait to participate in a serological survey in New Delhi. (Express Photo: Abhinav Saha)

What does it mean when so many are likely infected?

There is a silver lining from the lack of adequate testing. Progression towards an infection level where “herd immunity” begins to look plausible, it appears, has been faster than could have been possible if more tests were being conducted and more infected people were getting identified and isolated. The conventional wisdom is that greater testing and isolation slows down disease growth and delays the achievement of widespread immunity in the community. The caveat remains: Scientists still do not know the level of infection in the population at which “herd immunity” would start to play a role. But whatever level it is, India has been moving towards immunity at a community level at a faster rate than a capability for greater testing and isolation would have allowed.

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How do the serological findings relate with confirmatory test results?


The serological survey in Pune has some interesting findings. The ward in which the highest prevalence was detected is also the one that is now growing at the slowest pace in the last couple of weeks. In this area, Lohiya Nagar, more than 60% of the participants were detected with antibodies. It would still be premature to suggest that Lohiya Nagar could be near to achieving immunity at the community level, but scientists can now look at other areas that are approaching that level of infection to see whether the disease is behaving in similar fashion. As of now, there is insufficient data to make any conclusions.

There are a few other indicators as well. Cities with very high prevalence rates, like Delhi, Mumbai or Pune, have already begun to show signs of slowing down. The growth rates in Delhi and Mumbai have been declining for quite some time, while Pune is just beginning to join the trend. The reproductive number, or R, for these cities has gone below 1. R is the number of people who get infected, on an average, by an already infected person. An R-value lower than 1 would mean that, on an average, not every infected person is passing on the virus to others. Usually, the R-value going below 1 signifies the beginning of the decline of the epidemic, although in the present case one needs to be careful about making such conclusions since the pandemic is still on, information is still evolving, and R-values are only estimates.

But the drop in the growth rates, and in R-values, correlates well with what is expected when about half the population has already got the infection. The pool of uninfected people reduces progressively, and therefore the transmission rate goes down because of the availability of fewer people who can potentially be infected.

But is it not understood that immunity across a community is not a certainty?

Indeed, there are important caveats. The serological surveys only reveal the likely extent of disease prevalence in a population group. People who have been infected need not necessarily have developed immunity. As scientists have been insisting consistently, getting the infection and developing immunity are two different things. The serological survey finds out the number of people who have antibodies specific to the disease. The presence of these antibodies is an indication that they were infected at some point of time.


But immunity comes from what are known as “neutralising” antibodies, or “protective” antibodies. The serological surveys do not detect neutralising antibodies. For that, another test is required. And even though no case of re-infection has so far been detected, it is still an open question for investigation.

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Immunity at a community level is possible only if those who have been infected also develop immunity to the disease. As of now, we don’t know. That is the next step of investigation. Scientists are already preparing to carry out exercises where they would try to assess whether an infected person has developed immunity. Until then, discussions around “herd immunity” would not be substantive.

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First published on: 20-08-2020 at 03:34:18 am
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