Herd immunity: The curious case of Dharavi

The government and the medical fraternity need to think of ways to mitigate the virulence of the Coronavirus in the potential next wave. While personal hygiene, wearing of masks, maintaining social distance and universal vaccination are inevitable, the idea of herd immunity has once again gained prominence in the trending debate over whether to rely on herd immunity or on lockdowns. This has turned the spotlight on Dharavi— home to over a million slum-dwellers crammed into just over two square kilometres of area, where recently there have been periods of no Covid-19 cases reported.

According to our research, titled Dharavi Slums (Mumbai, India): The Petri Dish of Covid-19 Herd Immunity, which I co-authored with Zameer Shervani and Roma Nikhat of Food & Energy Security Research & Product Centre, Japan, and was published in late May in the European Journal of Medical and Health Sciences, the Dharavi population has demonstrated that antibodies acquired through herd immunity could keep Covid-19 at bay. It raises the question whether what worked in Dharavi could be replicated elsewhere and if herd immunity could be a viable alternative to lockdowns.

Dharavi is the antithesis of lockdown, which mandates avoiding crowded places, living in well-ventilated and sanitised places and maintaining the highest standards of personal hygiene — none of this is possible in Dharavi. By these parameters, the mortality rate should have been exponentially high there, but it has turned out to be just the opposite. It is a unique case of people developing high levels of immunity by total exposure to pathogens, resulting in rapid proliferation of infection and equally swift formation of antibodies in infected individuals. Also, we must remember that the vaccination rate remains low in Dharavi.

Herd immunity

Herd immunity, also known as population immunity, is the protection from an infectious disease that happens when a population becomes immune either through vaccination or through immunity developed through previous infection. Herd immunity against Covid-19 should be achieved by protecting people through vaccination, which the World Health Organisation (WHO) supports, and not by exposing them to the pathogen, attempting which could result in unnecessary cases and deaths. The percentage of people who need to be immune in order to achieve herd immunity varies with each disease. According to WHO studies, herd immunity against measles requires about 95% of a population to be vaccinated— the remaining 5% will be protected by the fact that measles will not spread among those who are vaccinated. For polio, the threshold is about 80%. The proportion of the population that must be vaccinated against Covid-19 for herd immunity to kick in is not yet known. This is an important area of research and will likely vary according to community, vaccine, populations prioritised for vaccination and other factors.

The first (July-August 2020) and the second (October 2020) serosurveys conducted in Dharavi showed that seroprevalence was found to be 57% and 45% respectively. Such a percentage of antibody occurrences must be good for herd immunity. In the other slums of Mumbai, such as in Cuffe Parade, seroprevalence was 75%. The same 75% population of Dharavi should also have developed antibodies by now. This is one of the main reasons for zero cases being reported there. But this cannot be recommended elsewhere unless the group or population is fully vaccinated. The risk of herd immunity becoming counterproductive due to the non-formation of antibodies is high.

Although the findings of the research will certainly help to set the parameters for another serosurvey in select areas or cities so as to assess the extent of antibody formation in the population, it is difficult to extrapolate the findings to be universally applicable. It cannot be applied in other areas as it is but it can be the base model for them.

Third wave

Experts working with WHO and a number of virologists have forecast a more serious and more virulent third wave. With high levels of antibody prevalence and vaccination rates, herd immunity may ward off the intensity of the third wave. We have sufficient experience in handling the pandemic and with a greater number vaccinated, increased testing, speeding up the production of necessary equipment and medicines, we could reduce mortality during the third wave.

It will be fatal to think that the pandemic is over and it is safe to resume a normal life. As of today, we don’t have complete knowledge of the characteristics and the specific structure of the Coronavirus. Moreover, we need to prepare our medical fraternity to deal with the virus’ mutations.

Wearing a mask, good and effective personal hygiene and developing high immunity are the standard operating procedures now. Incidentally, people in Dharavi used facemasks and remained confined due to the lockdown. Further studies should be conducted in areas where the population density is high but pandemic-related mortality was low. Also, we have to be alert about the rural areas and increase medical facilities there as the third wave may affect these areas more severely.

(The writer is with the Centre for Skin and Hair Pvt Ltd)