At the beginning of the COVID-19 pandemic last year, there was a lot of discussion on herd immunity in India and around the world.
Without vaccines, experts had warned against pursuing the herd immunity strategy to overcome the pandemic, saying it could to put people's lives at risk.
But now, we have vaccines and vaccination campaigns are being conducted in full swing. So, is it possible to achieve herd immunity against coronavirus now?
Herd immunity refers to the indirect protection from infection conferred to susceptible individuals when a sufficiently large proportion of immune individuals exist in a population.
Epidemiologist and Senior Advisor at Ashoka University, Dr. Lalit Kant, explains that there are two ways to be immune to an infection:
Immunity through natural infection
Immunity with the help of a vaccine
Dr. Harish Chafle, Senior Consultant Intensivist and Chest Physician, Global Hospital in Mumbai, says, "The first approach here can be considered an unethical approach to solving a problem because letting people contract the virus to attain herd immunity will lead to deaths of a significant population as well as adverse long-term health implications."
Dr. Lalit Kant says, "The key to herd immunity is that transmission of infection is blocked by immune persons around the susceptible or non-immune individuals."
Generally it is assumed that an infection gives a lasting immunity and so does a vaccine. However, Dr Kant thinks in the context of Covid-19, things are not that simple.
Dr. Chafle says the emerging variants of the virus result in increased infectivity. "Various factors, both constant and variable play part in the prediction of trends and estimations."
Reinfection can prove to be a main factor in preventing herd immunity. Dr. Kant explains that the immunity from an infection is of a indefinite duration.
Right now, it cannot be said with certainty whether people who have been infected with the covid and the antibodies that have formed in their body will be able to protect them from infection again.
How long does the immunity last?
A study published in the Lancet, said there is no evidence of permanent protective immunity to SARS-CoV-2 even after the coronavirus infection.
Dr Kant says that natural infection-induced immunity is unlikely to play a major role in the development of herd immunity.
Therefore, the option of vaccination is important to achieve herd immunity. But, it also has its own challenges.
Vaccine efficacies and percentage of the immunised population are key to herd immunity.
According to Dr Kant, the basic reproduction number R0 for SARS-CoV-2 is 2.5–3.5 – the number of individuals to whom an infected person is capable of spreading the infection.
Dr Kant explains, "Simulation exercises have revealed that to extinguish an ongoing epidemic, for a vaccine which has an efficacy of 60 percent would require 100 percent of its population to be immunized. The coverage level drops to 60 percent if a vaccine has 100 percent efficacy."
"For a vaccine which has less than 60 percent efficacy, herd immunity is unachievable and epidemic control is substantially poor," he adds.
Blocking transmission is pivotal to herd immunity.
Dr. Harish Chafle explains that the proportion of population that needs vaccination to achieve herd immunity against COVID-19 is not known.
In the initial studies, it was estimated that if 60 to 70 percent of the population is immune to Covid, then we can achieve herd immunity. According to the World Health Organisation, current estimates require 85 to 95 percent to be immune.
Several variants of COVID-19 have been identified around the world.
Newer variants of coronavirus may present challenges such as increased transmissibility, increased severity of the disease, decreasing antibodies in case of previous infection or vaccination.
He says there is no guarantee that more variants will not emerge in the future, which may further affect the effectiveness of the vaccine.
There has also been a decline in the efficacy of the vaccines against new variants compared to the original strain. However, protection against the virus still remains.
That's why experts clearly say that even though there is a possibility of reduced efficacy of the vaccine due to the variants, it does not mean that the vaccines don't work.
Cases of breakthrough infection have been reported even after taking two doses of the vaccine.
Cases of infection have been observed even after taking one or both doses of the vaccine. But studies have found that even if those who take a dose of the vaccine are infected, most do not develop serious illness.
According to Dr Uma Sekar, Associate Dean and professor of Microbiology at Sri Ramachandra Medical College and Hospital, the vaccines available do not offer full immunity towards the new mutations, The New Indian Express reported.
"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.
Therefore, it is being advised to follow the Covid protocols.
Dr. Harish Chafle also said that getting herd immunity is a challenge, but it is not because of the increased transmissibility of the new strain.
Vaccines prepare our bodies to fight viruses or other pathogens without making us sick.
In the case of the coronavirus vaccine, it has been reported that the vaccine can be helpful in protecting against infection, falling seriously ill if infected, and preventing death from the disease.
They say achieving herd immunity with safe and effective vaccines makes diseases rarer and saves lives.
According to Dr. Chafle, for now, we know with certainty that vaccinations, healthcare facilities and proper regulation are the only way to move towards herd immunity in India.
It is important that more people get the vaccine as soon as possible and we should not let our guard down anytime soon.