Vaccine and challenges.

The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the causative agent of coronavirus disease 2019 (Covid-19), has resulted in more than 74 million reported cases and over 1.6 million deaths worldwide since December 2019. The pandemic continues to devastate public health, the global economy and social order.

Safe, efficacious, and effective vaccine are urgently needed to halt the pandemic through the curtailment of SARS-CoV-2 infections, viral shedding, transmission among humans, disease severity, hospitalisations and deaths as well as to minimise post-infection complications and potential virus transmission from humans to certain animal species and vice versa.

The typical outcomes desired of a successful vaccination campaign include statistically significant reduction in infection and severity of consequential clinical disease or the duration of infectivity. The major goal of widespread vaccine deployment is to develop herd immunity, also known as herd protection. Typically, about 65 to 70 percent of the population in a geographic region needs to be immunised to exhibit herd immunity against a pathogen such as SARS-CoV-2.

The vaccine development against SARS-CoV-2, like any significant pathogen, is a costly, time-dependent and laborious process that can typically take more than a decade. It consists of the exploratory stage, preclinical stage, clinical stage, review and approval stage, post-licensure manufacturing-initially on a smaller and then on a mass scale-and post-marketing surveillance stage and quality control. The preclinical stage may involve in vitro studies in cell culture and in vivo studies in animal models such as mice, Syrian hamsters, ferrets and rhesus macaques. The clinical stage consists of three phases; Phase I which is safety testing in a small number of individuals, Phase II which is when safety testing is expanded from a few hundred to a few thousand individuals, and Phase III through which the vaccine efficacy testing I carried out usually in many thousand individuals.

Currently, more than 200 Covid-19 vaccines, using multiple platforms, are under development in various research facilities worldwide. It is remarkable that several vaccine candidates have entered or completed Phase III Clinical trials in less than a year. Recent approvals and licensure of Covid-19 vaccines from Pfizer and BioNTech, Moderna, and AstraZeneca-University of Oxford are based on statistically significant efficacy of these...