First signs of ‘herd immunity’ in small population groups in Pune

In a first-of-its-kind finding for any Indian city, a new study conducted in Pune has revealed that close to 85 per cent of the people who had been found infected with coronavirus in an earlier serosurvey had developed protective antibodies. In effect, they had acquired immunity from the disease.

The latest finding is the result of a follow-up study done in five prabhags (comprising three or four municipal wards each) of Pune where a serosurvey conducted earlier this year, in July and August, had found that, on an average, about 51 per cent of the people had been infected with the virus.

Serosurveys, like the ones carried out in several other cities as well, estimate the extent of the disease spread by detecting antibodies among surveyed population groups. However, the presence of antibodies only indicates that the person had been infected with the disease at some point of time. It does not mean that the infected person has also acquired immunity against the disease. Immunity comes only if the infected person develops what is called ‘neutralising’ or ‘protective’ antibodies, a small sub-set of antibodies.

This is the first study that followed up on an earlier serosurvey to detect the presence of ‘neutralising’ antibodies in infected persons. And though the researchers who carried out the study are careful not to suggest that the city was approaching ‘herd immunity’, this is the first documented case in the country where the infection rate in a population group had gone up so high that the concept of herd immunity could already be playing out.

This is also supported by the fact that in Lohianagar, the prabhag with the highest disease prevalence detected in the serosurvey, the incidence rate has fallen sharply in the last three months. This indicates that there may be population-level protection against the disease.

“I don’t think we can still say that Pune has achieved, or is reaching, herd immunity. But the study is important because it shows that wherever there was high sero-positivity, the incidence rate has fallen subsequently,” Dr Gagandeep Kang, one of the co-authors of the study whose preprint was released on Thursday, said. The study is still to be peer reviewed.

However, Kang added, “Pune residents must not relax or lower their guards, and crowd market places like we have seen in Delhi. If people do not protect themselves, a second wave in Pune can’t be ruled out. A lot of control is in our own hands, and a lot depends on how people behave.”

The level of protective antibodies in the infected persons also varied from individual to individual. “About 85 per cent of the infected people have shown detectable amount of neutralising antibodies. Of this, about 50 per cent had high levels of neutralising antibodies,” said Professor Aurnab Ghose, principal investigator of the study.

“We have found that there was a close association — low incidence of new diseases in areas of high sero-prevalence and vice versa. These are early observations, but may be an indication of population-level protection against Covid-19, at least in the short term,” he said.

Pune has so far reported 3.44 lakh confirmed cases of coronavirus infections, the third highest for any city, after Delhi and Bengaluru. These are the people who were tested and found to be positive. The city has also recorded over 7,200 coronavirus-related deaths, the maximum after Mumbai and Delhi.

Serosurveys estimate the wider prevalence rate, because not every infected person shows symptoms and gets tested.

Epidemiologist Jacob John, another co-author of the study, said if the current level of public behaviour continued, a second wave, the kind the US and some European countries are currently experiencing, was unlikely to happen in Pune.

“The serology suggests that in these populations, sporadic cases might happen, but another wave is unlikely. However, the entire Pune city will not behave in an uniform manner. We will have pockets of susceptibility, but the city on a whole will hopefully behave differently from Europe and the US because of this high sero-prevalence,” he said.

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