Effectiveness of and Immune Responses to SARS-CoV-2 mRNA Vaccines and Their Mechanisms

Following the online publication of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) genome in January 2020, two lipid nanoparticle-encapsulated mRNA vaccines, BNT162b2 and mRNA-1273, were rapidly developed and are now being used worldwide to prevent coronavirus disease 2019 (COVID-19). The mRNA of both vaccines encodes the full-length spike protein of SARS-CoV-2, which binds to the host cell receptor angiotensin-converting enzyme 2 and is believed to mediate virus entry into cells. After intramuscular injection of the vaccine, the spike protein is produced in the cells. Both humoral and cellular immune responses to the spike protein are elicited for protection against COVID-19. The efficacy of the two mRNA vaccines against COVID-19 with wild-type SARS-CoV-2 is more than 90% and is slightly decreased with the Delta variant, which is currently the predominant variant in many countries. In this review, the effectiveness of and immune responses to COVID-19 mRNA vaccines and their mechanisms are summarized and discussed. Potential waning immunity and an additional dose of COVID-19 mRNA vaccines are also discussed.