NEW DELHI: A day after the central government asked the states to conduct sero surveys to understand the extend of coronavirus infection among the population, the lingering question is whether Delhi has achieved herd immunity against the novel virus. The last sero survey in Delhi was conducted in February, according to which 56.13 per cent of the city’s population had antibodies against the virus.
This was when the city was seeing daily cases in the range of 100-300. The high prevalence of antibodies in the survey was not surprising since the city had witnessed as many as 8,500 daily cases in November 2020. Based on this result, some experts suggested that Delhi had achieved herd immunity.
However, the optimism was short-lived and the city was devastated by a much fatal wave of the pandemic which lasted for over a month. Does this mean that the concept of herd immunity have some flaws? Experts say there are three main factors transmission rate of virus, susceptible population, and also to a large extent the variant of the virus that decide how a pandemic wave is going to impact the population
“Herd immunity is based on the type of pathogen. The infection and fatality rates determine how fast the virus is spreading. If we conduct a sero survey, it won’t show which variant of the virus produced the antibodies. The survey will show the effect of all variants, new and old alike,” said Dr Jugal Kishore, head, Community Medicine at Safdarjung Hospital.
“The first and second sero surveys gave small figures but the last one was above 50 per cent. This showed that we were close to achieving herd immunity. However, when a new variant comes, whoever is left becomes susceptible,” added Dr Kishore.
“Irrespective of the variant, the proposed survey will say what proportion of the population has antibodies,” said Dr Chandrakant Lahariya, public health expert and author of the book Till We Win: India’s Fight against the Covid-19 Pandemic. Dr Lahariya is of the opinion that herd immunity is not applicable in small areas.
“Also, viruses and mutants are different. The nature of protection ranges from six to nine months. And Delhi may witness another wave much before it happens at the nat ional level," Dr Lahariya added.
“If the virus is killing large number of people, then herd immunity won’t occur. The new variant was spreading very fast. If the infection was slower, the herd immunity up to 70 per cent could have been achieved. Those who are left out should be vaccinated first because they are more prone to the infection,” added Dr Kishore. Speaking about the possibility of next wave, Dr Kishore said, “Youths and small children are at higher risk if the next wave set in.”
5th survey (February)
56.13 per cent
of the 28,000 people sampled during Delhi’s fifth serological survey had antibodies against Covid-19
40.09 per cent
Lowest sero prevalence in the North district
62.18 per cent
Highest sero prevalence in the Southeast district
All the other 10 districts in Delhi had a sero prevalence of more than 50 per cent
6/10 districts
Had sero prevalence more than 55 per cent