Current status and prospect for development of COVID-19 vaccine adjuvants

Vaccines are the most economical and effective means to protect against SARS-CoV-2 infection. Adjuvants are able to enhance antigen-specific immune responses, reduce antigen doses and vaccination times, and reshape adaptive immune responses, which are crucial to improving the protective efficacy of vaccines. In this article, we review the mechanisms and advantages/disadvantages of the adjuvants used in COVID-19 vaccines that have been authorized for emergency use or are undergoing clinical trials around the world, and analyze the issues that must be considered in the application of adjuvants to COVID-19 vaccines. In addition, we put forward suggestions for future research strategies for COVID-19 vaccine adjuvants: ① adjuvants for immunocompromised people, ② adjuvants for inducing T cell responses, ③ novel mucosal immune adjuvants, and ④ the rational design of combination adjuvant.