While the available coronavirus disease 2019 (COVID-19) vaccines have demonstrated good efficacy in reducing transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), the ongoing global demand and risk of variants of concern call for the continued generation of new vaccines.
Vaccine efficacy is measured by several correlates of protection, chiefly the response of neutralizing antibodies in the sera of the vaccinated towards SARS-CoV-2 in vitro. However, other aspects of the immune response in vivo, such as T-cell involvement, are difficult to study in the early stages of vaccine development owing to safety concerns relating to human testing.
