Promises and challenges of mucosal COVID-19 vaccines.

Coronavirus disease-2019 (COVID-19) is an ongoing pandemic caused by the newly emerged virus severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Currently, COVID-19 vaccines are given intramuscularly and they have been shown to evoke systemic immune responses that are highly efficacious towards preventing severe disease and death. However, vaccine-induced immunity wanes within a short time, and booster doses are currently recommended. Furthermore, current vaccine formulations do not adequately restrict virus infection at the mucosal sites, such as in the nasopharyngeal tract and, therefore, have limited capacity to block virus transmission. With these challenges in mind, several mucosal vaccines are currently being developed with the aim of inducing long-lasting protective immune responses at the mucosal sites where SARS-COV-2 infection begins. Past successes in mucosal vaccinations underscore the potential of these developmental stage SARS-CoV-2 vaccines to reduce disease burden, if not eliminate it altogether. Here, we discuss immune responses that are triggered at the mucosal sites and recent advances in our understanding of mucosal responses induced by SARS-CoV-2 infection and current COVID-19 vaccines. We also highlight several mucosal SARS-COV-2 vaccine formulations that are currently being developed or tested for human use and discuss potential challenges to mucosal vaccination.