Vol. 6, Issue 10, Part B (2020)

Abstract
During current Covid19 pandemic, herd immunity has become a hot topic. In absence of vaccine and specific drug(s) against the virus, it is only the ‘herd immunity’ towards which we can look for respite from the pandemic. The present paper uses Mathematical formula of ‘heard immunity threshold’ in completely susceptible population, (as in case of Covid19 pandemic) = [1 – 1/R₀], and finds that in India for achieving herd immunity against Covid19 virus, at least 75% of its population is to be exposed to the virus. Thereafter, increment in proportion of infected population in the country over six days is estimated and date of achieving 75% of infected population is arrived at through extrapolation, which is found to be 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. The ascending epidemic curve will start downward slide weeks before 17 November, 2020 and gradually India will come out of the pandemic unless a second wave of pandemic due to mutation of Covid19 virus, for the worse, comes. In the whole exercise a few reasonable assumptions are made. It is immensely expected that the paper might stimulate Epidemiologists to fine-tune the methodology to estimate more valid result in respect of herd immunity in India. There is an issue of re-infections with same Covid19 virus due to early decline of anti-body level in some persons during the present pandemic. These re-infections are seen in a few cases in India. This issue is not significant in the present study as re-infections are asymptomatic or very mild in nature. These will not change the broader picture of the pandemic. But early decline of antibodies may cause problem at another level. It may delay the development of precise heard immunity and Covid19 may continue as low endemic disease in India till a vaccine is developed.