The story so far: Right from the time it became obvious that the SARS-CoV-2 virus outbreak was not confined to China, but scaling other shores too, the theory of herd immunity has been floating around. Initially, herd immunity, an important tool in epidemic control, was proposed as a means to overcome the pandemic. Only a certain proportion of the population needs to be infected in order to stop large outbreaks, either through naturally-acquired disease, or through vaccination. Since a vaccine is not available for COVID-19 yet, some people advocated that the infection be allowed to spread in the community until herd immunity is achieved.

Why is this being stoutly opposed?

The SARS-CoV-2 virus is easily transmissible and would require around 60-70% of the population to be infected to acquire herd immunity.

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In the World Health Organization’s (WHO) ‘Science in 5’ video series, the organisation’s chief scientist, Soumya Swaminathan, explains herd immunity with the help of a common childhood infectious disease — measles — for which there is a very effective vaccine. “To achieve herd immunity in the population, for measles, you need about 95% of the people to have immunity or antibodies,” she explains. “Even if you have 5% of children [who are] not vaccinated, these others actually have enough protection in the population to prevent the measles virus from actually going from one person to the next. So, it is really like having a barrier of people who are protected, who break that chain of transmission.”

For the SARS-CoV-2, Dr. Swaminathan thinks that at least 60-70% of the population should have immunity to really break the chain of transmission. “If you allow this to happen naturally, it will take a long time, of course, but more importantly, it is going to do a lot of collateral damage,” adds Dr. Swaminathan. “So, even if 1 % of people who get infected are ultimately going to die, then this can add up to a huge number of people, if we look at the global population. And that is why we believe [that] it is not a good idea to achieve herd immunity by just letting the infection run wild in the general population and infect a lot of people. We should talk about herd immunity in the context of a vaccine,” she says.

Marc Lipsitch, Professor of Epidemiology and director, Center for Communicable Disease Dynamics, Harvard School of Public Health, responding on Twitter to the news of the White House’s new pandemic adviser recommending ‘herd immunity’, said: “This is simply wrong … Herd immunity is not a strategy or a solution. It is surrender to a preventable virus.”

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How and when will herd immunity be achieved in this pandemic?

Arnaud Fontanet and Simon Cauchemez, authors of a recent Nature publication, ponder over this exact point: “Herd immunity is achieved when one infected person in a population generates less than one secondary case on an average, which corresponds to the effective reproduction number R (that is, the average number of persons infected by a case) dropping below 1 in the absence of interventions.” The publication says, “With flu pandemics, herd immunity is usually attained after two to three epidemic waves, each interrupted by the typical seasonality of influenza virus, and more rarely, by interventions, with the help of cross-protection through immunity to previously encountered influenza viruses, and vaccines, when available.” Currently, there is insufficient confirmed information on re-infection to determine how that will influence the pandemic’s course.

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What does the sero-prevalence study in India say?

A measure of the infection levels in the population is provided through the sero-surveys conducted by the Indian Council of Medical Research (ICMR). The second study was done between August 17 and September 22, 2020, and found the prevalence at 15.6% of the population in urban slums and at 8.2% in non-slum areas. The hotspots were not included this time. In Delhi, the second round estimated a sero-prevalence of 29.1%. The study revealed that for every reported COVID-19 case, there were 26-32 infections, down from 81-130 infections per reported case in May. It also said the susceptibility of a considerable section of people, who are still unexposed to SARS-CoV-2, exists, and the risk in urban slums is twice that in non-slum areas, and four times higher than the risk in rural settings. In Mumbai (study not by ICMR), the second-round estimated around 45% prevalence in slums and 18% sero-prevalence in non-slums.

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Are vaccines the solution?

The Nature paper argues that vaccines are particularly suited for creating herd immunity because “their allocation can be specifically targeted to highly exposed populations, such as healthcare workers or individuals with frequent contact with customers.” They may have a significantly greater impact on reducing viral circulation than naturally-acquired immunity, especially if it turns out that naturally-acquired protective immunity requires boosts through re-infections.

Dr. Swaminathan says, “With a vaccine you can achieve immunity and herd immunity safely. Through natural infection also we can achieve it at some point, but it would be at great human cost.” Meanwhile, until a vaccine is readied, it would be best to take actions that would slow down the progress of the virus — following COVID-19 etiquettes of wearing a mask, physical distancing and following hand hygiene — she adds.