Herd Immunity Threshold is not the 80% described by epidemiologists and mathematicians. It is instead more between the range of 10% to 20%.
This is the reason that substantial decline in deaths have occurred in locations such as London, New York and Stockholm with about 20% of population displaying evidence of infection.
Populations are heterogenous in nature, there are people of different age, sex, physical fitness, susceptibility etc. In the real world humans are not identical nor predictable as envisaged by theorists.
Delhi as per reports available already has more than 5% people infected, and if we take into account that twice this number also has killer T cells we have a figure of almost 15% infected, in other words we are rapidly approaching herd immunity threshold.
It has been founds that almost 80% of people not infected by Covid-19 in the past are capable of carrying T cells (from probably previous common colds) that react against the new Coronavirus.
The jury is still out concerning airborne transmission; but not would be prudent to avoid crowds, closed spaces, and keep a little distance from people. Aerosols are less than 5 microns in size.
