CHENNAI: In early 2020, during the initial phase of the Covid pandemic in India, a lot was discussed about herd immunity and how it would drastically bring down the number of cases.
Over a year later, we are still far from achieving it. What are the complexities of achieving herd immunity against Covid and why a great deal remains to be done?
Why is it important?
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.
When enough people (threshold population) have immunity against Covid, it would significantly reduce the rate of spread and contain transmission within smaller pockets of location. Effectively, it will restrict the pandemic.
In Tamil Nadu, about 23 lakh people have tested Covid positive and almost 22 lakh have been vaccinated. This means that only about 6.4 per cent of the seven crore people in the State have immunity against Covid. “We need at least four crore people in the State to be vaccinated, in order to approach herd immunity,” said Dr P Kuganantham, senior epidemiologist.
Early studies estimated that if 60 to 70 per cent population was immune to Covid, we would reach herd immunity. This has changed since. The current estimates require 85 to 95 per cent of people to become immune, according to the World Health Organisation. What caused this change?
The mutating virus
During the initial days of the pandemic, not only did experts think that herd immunity will be reached at 60 per cent, but also thought the transmission rate will drop rapidly after 40 per cent of the population became immune, said Dr H Kolandasamy, former Director of Public Health of Tamil Nadu.
“What they did not account for was the unusually high transmission rate of the disease,” he added.
Large segments of the population would have gained immunity if the disease was allowed to spread uncontrolled.
However, this would have not only increased the number of deaths in a short period of time, but also increased the mortality rate itself due to overburdening of the system. So, mass vaccination drives and controlling of virus spread using lockdowns were the best options available to us.
For a substantial portion of the population to be vaccinated, three factors play a crucial role; strategy for rolling out vaccine drives, efficacy of the vaccine and public’s willingness to take it. We had effectively curtailed Polio through a successful vaccination drive.
However, Polio virus mutates very slowly compared to the novel coronavirus.
With newer Coronavirus variants on the rise, which are suspected to be more transmissible, estimating a threshold population for herd immunity and achieving it has become tough, opined Dr Uma Sekar, Associate Dean and professor of Microbiology at Sri Ramachandra Medical College and Hospital.
Incomplete vaccine data
When the first vaccines against Covid were approved, many experts globally expected populations to achieve herd immunity within a few months.
“Vaccines available do not offer full immunity towards the new mutations. So we have some cases of breakthrough infections, wherein a vaccinated individual is asymptomatic or mildly symptomatic, but continues to spread the disease,” said Dr Sekar.
Further, even though in small numbers, Tamil Nadu has observed breakthrough infections when a vaccinated person tests positive for the infection.
“Many who were infected in the first wave contracted the virus again. This means that we are yet to ascertain if immunity from both vaccination and infection is long term. We do not know if short term immunity will lead to herd immunity,” said Dr Ashwin Karuppan, Senior Consultant, Internal Medicine, Gleneagles Global Health City.
The way forward
Dr Karuppan also said that those vaccinated and tested Covid positive, however, had mild to no symptoms.
“Covid may not disappear and may continue to exist in pockets. But, with more data and
enhanced vaccination drives, we may be in a better position to deal with it in the future,” he said.
Dr Kolandasamy, however, added that despite the poor vaccination rate and rapid mutations, four characteristics of the virus have remained constant.
“The mode of transmission of the virus, diagnostic technology, the preventive methods and treatment remains the same for all variants. As far as public health is concerned, the strategy to fight the disease has not changed at all. Slow the disease transmission until the threshold population is vaccinated,” he concluded.
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