Herd immunity in Delhi? Sixth sero survey reveals more than 90% have antibodies against Covid

New Delhi, Oct 28: More than 90 percent of those covered under the sixth serological survey in Delhi have developed antibodies against coronavirus, according to a report submitted to the government on Wednesday.

This means that Delhi is unlikely to witness a Covid wave as destructive as the second wave in April and May until a new severe variant emerges, a government official said.

"However, we cannot say Delhi has achieved herd immunity despite such a high level of sero-prevalence," the official said.

Also, it cannot be said whether vaccination has a role to play in the high seropositivity rate in the capital.

"We have found Covid antibodies in more than 90 per cent of the samples collected during the sixth round of the survey," an official source said.

The sample collection under the sixth sero-survey started on September 24.

A total of 28,000 samples were collected from all the 280 wards, including New Delhi Municipal Council and Cantonment Board wards, in the capital.

The seropositivity rate in every district is more than 85 per cent.

More women were found seropositive as compared to men, sources said.

"We cannot say exactly if Delhi has achieved herd immunity because there are no studies or data to show that the virus won't spread after a certain percentage of population is affected," the official quoted above said.

"However, the same variant of the virus (Delta) cannot lead to a major epidemic. So, we can say there won't be another wave of coronavirus as big as the second wave until a new severe variant emerges," the official said.

Also, the antibodies developed will be partially effective against the new variant.

This is the first survey to be done after Delhi was hit by a fourth Covid wave in April and May.

The fifth round of sero survey conducted in January had shown that 56.13 per cent of the people in Delhi had developed antibodies against COVID-19.

In April and May, Delhi battled a brutal second wave of the pandemic that claimed a massive number of lives with oxygen shortage and reduced supplies of essential medicines at hospitals adding to the woes.

On April 20, Delhi had reported 28,395 cases, the highest in the city since the beginning of the pandemic.

On April 22 the case positivity rate was 36.2 per cent, the highest so far.

The highest number of 448 deaths was reported on May 3.

What is 'herd immunity'?

'Herd immunity', also known as 'population immunity', is the indirect protection from an infectious disease that happens when a population is immune either through vaccination or immunity developed through previous infection. WHO supports achieving 'herd immunity' through vaccination, not by allowing a disease to spread through any segment of the population, as this would result in unnecessary cases and deaths.

Herd immunity against COVID-19 should be achieved by protecting people through vaccination, not by exposing them to the pathogen that causes the disease.

To safely achieve herd immunity against COVID-19, a substantial proportion of a population would need to be vaccinated, lowering the overall amount of virus able to spread in the whole population. One of the aims with working towards herd immunity is to keep vulnerable groups who cannot get vaccinated (e.g. due to health conditions like allergic reactions to the vaccine) safe and protected from the disease.