How Will the Covid-19 Vaccines Help Shape the Herd Immunity against Covid-19?

Abstract

Severe acute respiratory coronavirus-2 (SARS-CoV-2 and the related disease COVID-19) has revealed the devastating impact of the pandemic on a susceptible human population. Herd immunity is considered as an effective intervention to restrict the viral spread based on the conception that the proportion of individuals immune to SARS-CoV-2 is greater than the herd immunity threshold at which persistent transmission is unlikely to occur so that the pandemic is likely to decline. Herd immunity threshold varies with environmental, epidemiological and immunological settings. Establishment of herd immunity against SARS-CoV-2 within a population can be achieved by two approaches. The first is a large scale of vaccination that demands a safe and effective vaccine. The principles of vaccination against COVID-19 stem from cross-reactive immunity based on evidence from the pre-existing cross-reactive immune memory to SARS-CoV-2 that primarily originates from previous exposure to common cold coronaviruses circulating in the human population. The second is natural recurrent infections, the consequences of which are more serious and far-reaching at the cost of infection of a large proportion of the human population and death of the susceptible people. In the absence of the vaccines, many countries have implemented social distancing mandates and quarantine strategies to limit transmission and lower the infection rate; but another solution is effective vaccination, both of which facilitate herd immunity. A COVID-19 vaccine BNT162b1 has been developed, which is a lipid nanoparticle-formulated nucleoside-modified mRNA coding for the receptor-binding domain of the spike protein on the SARS-CoV-2 virion. The phases I/II trial data indicate that the vaccine is safe with mild side effects and induces both antibody and T cell responses, making it promising for phase III trial. Conclusions from the preliminary phase III data demonstrate efficacy of this candidate. Here we intend to provide insights about the role of vaccination in development of herd immunity and perspectives of the mRNA-based vaccine candidate in the context of pandemics.