Fig. 2: Evolving scenarios for global COVID-19 vaccine development and demand.

In response to the urgent demand for a vaccine, more than two dozen candidate vaccines are advancing through clinical trials following an expedited pandemic vaccine development paradigm, with many steps of the development process occurring in parallel before a successful outcome of previous steps has been confirmed. Vaccine candidates will continue to be preclinically and clinically evaluated following conventional and/or rationalized vaccine development processes over the next few years. These efforts will evolve to meet the demands for vaccination in several likely scenarios that are predicted on the basis of sociopolitical challenges and the emerging data regarding the trajectory of the coronavirus disease 2019 (COVID-19) pandemic and the host response to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). One scenario is the priority vaccination of high-risk populations such as health-care workers, seniors, people with co-morbidities and ethnic minorities, who have been disproportionately affected by COVID-19, when vaccine supply is initially limited. Aside from these prioritized groups, it may also be necessary to consider that asymptomatic individuals, patients who have recovered from COVID-19 but generated poor immunity or whose immunity quickly waned, and individuals who received a rapidly developed ‘pandemic’ vaccine that provided suboptimal protection or rapidly waning immune responses may require a booster vaccination to ensure sufficient levels of population protection for herd immunity. Ultimately, regional, continental and global populations will be subject to mass vaccination programmes based on the extent of national and global vaccine distribution and also likely according to the relative regional severity of outbreaks.