ABSTRACT
The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) that originated from the Wuhan district of China in Dec 2019 continues to spread worldwide infecting more than 173 million people in more than 200 nations and causing nearly 3.7 million deaths. Multiple waves of the coronavirus disease-19 (COVID-19) pandemic has inflicted severe economic injury and disrupted the health and educational infrastructure of the nations. So, the question that intrigues both the scientific community and the general population is whether “herd immunity” or “population immunity” can be achieved for SARS-CoV-2 by vaccination or by natural infection so that resurgences can be prevented in future. Eradication of small pox and polio, and elimination of measles have been achieved by attaining the herd immunity threshold, but SARS-CoV-2 is different from the others in terms of a lasting protective immunity post-infection and post-vaccination, emergence of immune evading variant strains and facilitation of transmission through asymptomatic cases. In this chapter, we focus primarily on the features of the basic Reproduction number (R0), and herd immune threshold unique for SARS-CoV-2 and the hurdles in attaining herd immunity using the currently available COVID-19 vaccines. Updated scientific evidence will help understand complexity in achieving herd immunity from COVID-19.
