Herd Immunity and Covid19 in India

Introduction

For some infectious diseases, herd immunity can go into effect when 40 percent of population is exposed1. But in most cases 80 to 95 percent of population must develop immunity to stop its spread. In case of Measles, 95 percent vaccination is required for developing herd immunity [1].

During current Covid19 pandemic heard immunity has become a very important consideration. As no vaccine and/ or specific drug(s) against the virus are available presently, it is only the herd immunity which is looked at for getting relief from the pandemic. Experts estimated that in the USA, Conceptual Paper

70 percent of the population would have to recover from COVID-19 to halt the epidemic [2]. Under the simplest formula, the development of heard immunity in Covid19 novel virus is a function of R0 (called ‘R naught’ and signifies basic reproduction number, that is average number of secondary infections caused by a single infectious individual introduced into a completely susceptible population).

Mathematically ‘heard immunity threshold’ (the proportion of population to be infected to interrupt transmission of an infectious disease) in completely susceptible population, as in case of Covid19 pandemic, is represented by formula [3] = [1 – 1/R0]. In USA, the experts placed the country’s ‘herd immunity threshold’ level at 70 percent, because initial R0 in USA was surely about 3.3. Thus ‘herd immunity threshold’ = [{1 – (1/3.3)} x 100] = 70 percent was found for USA. The present paper attempted to estimate the ‘herd immunity threshold’ level in India and tried to forecast as to when India could expect to achieve its ‘heard immunity threshold’ against Covid19 virus.

Methodology

India reported 10,815 cases [4] on 14 April 2020. For India, R0 was 1.83 during April this year [5]. It can be assumed that during beginning of Covid19 pandemic in India (February-March 2020), its R0 was 2. That way, ‘herd immunity threshold’ level in India would be 50 percent (by using above mentioned formula of [1 – (1/2) x 100]. But keeping in mind the lowering effect on R0 due to nation-wide complete lockdown for continuous 68 days [6] from 25 March to 31 May 2020, we can add additional 25 percent to India’s ‘herd immunity threshold’ level to reach a compromise of 75 percent.

In a sero-survey report [7] of Indian Council of Medical Research (ICMR), it was found that in first week of May this year, there were about 6.4 million of Covid19 infected persons in the country against reported cases of 52,592 on 7th May, 2020. That made the presence of about 122 Covid19 infected persons against each reported case in the country on 7th May 2020. Compromising for the issues of ‘sensitivity’ and ‘specificity’ of diagnostic tool(s) used, as well as, ‘confidence interval’ related to sampling methodology of ICMR study, it could be reasonably assumed that for each reported case of covid19 in India, there were 100 more unreported infected persons in the country.

Applying the assumption of infected unreported persons in India made above, the population percentage of Covid19 infected persons was estimated from 19 September to 25 September 2020 at 10:30 hours every day. The formula used was: p = [c + {c x m}/P x 100]; where ‘p’ = population percentage of Covid19 infected, ‘c’ = total reported cases on the day, ‘P’ = population of India on the day and ‘m’ = multiplication factor, which is 100 (number of additional infected persons per reported case). And then the increment in the percentage of Covid19 infected Indian population over those six days was extrapolated to find when India will reach 75 percent of infected population against Covid19 virus.

Results

From the below (Table 1), it is found that population percentage of Covid19 infected persons increased from 38.80 percent to 42.46 percent in six days. In other words, increment in population percentage of Covid19 infected persons of 3.66 percent (42.46 – 38.80) occurred in six days. So, further increment of 32.54 percent (75.00 percent – 42.46 percent) will occur in next [(6 x 32.54) / 3.66] = 53 days, that is on 17 November 2020 [8, 9].

Discussions

India is expected to reach its ‘herd immunity threshold’ level of 75% against Covid19 virus on 17 November 2020. But since ONE such day can’t be pin-pointed in any massive community-based biological situation, it is better to say that ‘around 17 November, 2020’ India will expectedly achieve herd immunity against Covid19 virus. In the preceding weeks of that date, the current ascending trend of epidemic curve in India will start its downward journey. And after around 17 November, 2020, India will be expected to come out of current pandemic gradually, unless attack of a second wave of pandemic due to mutation of Covid19 virus of worse pathogenicity comes. Two clades of Covid19 virus, namely A2a and A3i are now most dominant in India [10].

Pragmatically speaking, we in India have to keep our fingers crossed at the moment. It is highly important to continue with district-based monitoring of Covid19 situation. This has been emphasized in a recent study of same author [11] to keep a constant track of the situation to implement appropriate containment measures in required districts/ places.

India is a vast and populous country. So there will be variations in status of achieving ‘herd immunity threshold’ levels among districts and states. The forecasting methodology used in the study was simple and supported by a few reasonable assumptions. The result derived from the study is expected to give near-true picture in given situation. The study can stimulate Epidemiologists to fine- tune the methodology to get more valid estimate about ‘herd immunity threshold’ against Covid19 virus and expected date of achieving that in India.

Issue of re-infection due to declining antibodies needs to be addressed here. As the Covid19 pandemic continues, re-infections are found after few weeks to few months of first infection in some individuals across the world. India, too, has reported a few cases of re-infection [12]. It should be noted that in all the cases of re-infection reported till date, patients did not possess enough antibodies to fight the re-infection [12]. When antibodies tend to wane over time, the immune system doesn’t necessarily have to start again from scratch to fight re-infection. Memory of a past infection can be stored in a cell called a memory B cell. Yet another line of defense is a kind of immune cell called a T cell. Thus, there are many arms with which the immune system can tackle this re-infection [13].

Re-infections are common with other Corona viruses. So, re-infection with Covid19 is expected one. It may be possible to become infected again by same Covid19 virus. But the resulting infection will be mild or asymptomatic, with significantly lower levels of virus replication and onward transmission [14].

A sero-survey done in Delhi between 27 June and 10 July 2020 has found antibodies against Covid19 virus in 23 percent of surveyed population. Not surprisingly, in 97 persons out of 208, who had previously tested positive, no antibody against Covid19 virus could be found [15]. In other words, among 47 percent [{97 x 100} / 208] of previously infected people in Delhi, the antibody waned of.

In another round of sero-survey done in Delhi by ICMR during first week of August 2020, about 29 percent of surveyed people showed antibodies against Covid19 virus [15]. This gives a low level of estimate about Covid19 infected population in Delhi than what has been found in the present study in context of whole India. But, practically speaking, 47 percent of earlier sero-positives have memory B cell and immune T cell against Covid19. So this 47 percent should be added with 29 percent of later sero positives. Consequently, the ‘virtual sero-positives’ in the last sero-survey in Delhi would be 76 percent (29 percent + 47 percent). We know that this 47 percent contributes to herd immunity, but can’t objectively say anything about its quantitative effect. If we assume its effectiveness to be 20 percent, then ‘actual sero- positivity’ during last survey in Delhi (done in first week of August 2020) would be 38.4 percent (29 percent + 9.4 percent), which is almost similar to the findings of present study.

Re-infection with same Covid19 virus may not be very important from the view point of outcome and number. But re-infection can delay in development of clear-cut herd immunity. This may allow Covid19 to continue as low endemic disease in the country, till a potent and safe vaccine against it is developed. But around 17 November 2020, it is expected that there will be a major paradigm shift, for the better, in the present Covid19 pandemic situation in India.

The report of sero-survey [7] in India done in the first week of May 2020 by ICMR was taken as basis in the present study. Because at that initial period of pandemic issue of declining antibodies was not expected to be present. The present study took the course of a linear simplistic scenario. The actual situation must be more complex. However, if it can stimulate discussions among Epidemiologists, the purpose will be served. Meanwhile, optimism is the only virtue left with us.