Results of serological survey in India

A serological survey in Pune has provided new evidence that the spread of Covid-19 in India has been much greater than what is detected by confirmatory tests. In Pune, more than 51% of those who have been tested have shown the presence of antibodies specific to the new coronavirus, suggesting that around half of the population of this city of 4 million people may already be infected.

By the time the serological survey was conducted between July 20 and August 5, less than a lakh of people in the city had been found infected during confirmatory testing. Therefore, the actual spread could be 20 times greater than what confirmatory tests detected.
Previous serological surveys in Delhi and Mumbai have also given similar results. A previous exercise in Delhi had suggested that the actual spread could be 40 times the number of confirmed cases. A new round of survey in the national capital, the results of which have not yet been officially published, confirms this.

Results of serological survey in India: what is the end result?

These tests have been woefully adequate, despite a huge increase in the testing infrastructure. From the ability to test just a few hundred samples at the start of the outbreak in March to over eight lakh of testing per day now, there has been a massive upgrade in testing infrastructure. And yet, serological tests show that the vast majority of those infected are still excluded, especially those who have no symptoms.
Appropriate testing is essential for containment strategies. This is the only method to identify and isolate infected people and their close contacts.
The more tests are done, the better the chances of detecting infected people, including those who are asymptomatic.
Timely isolation of these can prevent transmission to other people. Therefore, more tests directly influence the slowing of the spread of the disease.
Given the current extent of the disease, based on the results of seroprevalence surveys, the screening capacity should be increased several times to make a significant difference in the growth of the disease.
It probably won’t happen overnight. So, while it would still be important to keep testing as many people as possible, you’ve passed the stage where testing was the most effective tool for flattening the growth chart.

What does it mean when so many people are susceptible to infection?

There is a silver lining in the absence of proper testing. The progression to a level of infection where “herd immunity” begins to sound plausible, it seems, has been faster than it would have been possible if more tests had been done and more tests were carried out. infected people were identified and isolated.
The conventional wisdom is that increased testing and isolation slows disease growth and delays the achievement of widespread immunity in the community.
The caveat remains; Scientists do not yet know the level of infection in the population in which “herd immunity” would begin to play a role. But regardless of the level, India has moved towards community-level immunity at a faster rate than the ability to conduct further testing and isolation would have allowed.

How do serologic results relate to confirmatory test results?

The serological survey in Pune has some interesting results. The room in which the highest prevalence was detected is also the one that is currently growing at the slowest rate in recent 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 close to achieving community-level immunity, but scientists can now look at other areas that come close to that level of infection to see if the disease behaves in the same way. At the moment, there is not enough data to draw conclusions.
There are also other indicators, cities with very high prevalence rates, like Delhi, Mumbai or Pune, have already started to show signs of slowing down.
Growth rates in Delhi and Mumbai have been declining for some time, while Pune is just starting to join the trend.
The number of breeders, or R, for these cities has increased from R1 is the number of people infected, on average, by one person already infected. An R-value of less than 1 would mean that, on average, all people infected.
There are also some other indicators cities with very high prevalence rates, such as Delhi, Mumbai or Pune, have already started to show signs of slowing down. Growth rates in Delhi and Mumbai have been declining for some time, while Pune is just beginning to join the trend.
The number of breeding animals, or R, for these cities has dropped below 1. R is the number of people infected, on average, by an already infected person.
An R value of less than 1 would mean that, on average, not all infected people transmit the virus to others. Generally, an R value of less than 1 signifies the beginning of the epidemic’s decline, although in this case care must be taken to draw conclusions, as the pandemic is still ongoing, the information is still evolving and the R values they are only estimates.
But the decline in growth rates and R values is in line with what is expected when about half the population has already contracted the infection.
The pool of uninfected people is gradually shrinking and therefore the transmission rate is decreasing due to the availability of fewer potentially infected people.

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

In fact, there are important caveats. Serological surveys reveal only the likely extent of disease prevalence in a population group.
Infected people do not necessarily have to have developed immunity.
As scientists constantly insist, getting the infection and building immunity are two different things.
The serological survey allows to know the number of people who have specific antibodies against the disease.
The presence of these antibodies is an indication that they have been infected at some point.
But immunity comes from so-called “neutralizing” antibodies or “protective” antibodies.
Serological studies do not detect neutralizing antibodies. For this, another test is necessary.
And although no case of reinfection has been detected so far, the matter remains open for investigation.
Immunity at the community level is only possible if those who have been infected also develop immunity to the disease. At the moment, we don’t know.
This is the next step of the investigation. Scientists are already preparing to perform exercises in which they would try to assess whether an infected person has developed immunity.
Until then, debates on “herd immunity” would not be substantial.