Reaching Herd Immunity for Covid-19, How and When?

Herd Immunity or is better known as Herd Protection happens when there is a sufficient proportion of immuned people in a particular population who will not be infected and cannot transmit a certain pathogen to the others who are susceptible.

The threshold or the proportion required to achieve heard protection can be calculated using this formula:

1-1/R

R= infectivity (the average number of cases infected by an infectious individual in an otherwise fully susceptible, well-mixed population). R changes with the changes in human and viral behaviours.

For example, measles is extremely infectious, with an R between 12 and 18, which works out to a herd-immunity threshold of 92–94% of the population.

The threshold is dynamic, changes with human behaviours and virus mutation. The actual threshold is only established in retrospect after about a decade of observation/stabilisation.

No herd immunity is possible if immunity from natural infection or vaccination does not last and reinfection occurs. Until an effective vaccine is available or regular revaccination is done, we will only enjoy herd immunity.

https://www.nature.com/articles/d41586-020-02948-4