The remarkable success of messenger RNA (mRNA) vaccines against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has propelled the rapid development of this vaccination technology in recent years. Over the last three decades, numerous studies have shown the considerable potential of mRNA vaccines that elicit protective immune responses against pathogens or cancers in preclinical studies or clinical trials. These effective mRNA vaccines usually contain specific adjuvants to obtain the desired immune effect. Vaccine adjuvants traditionally are immunopotentiators that bind to pattern recognition receptors (PRRs) of innate immune cells to increase the magnitude or achieve qualitative alteration of immune responses, finally enhancing the efficacy of vaccines. Generally, adjuvants are necessary parts of competent vaccines. According to the existing literature, adjuvants in mRNA vaccines can be broadly classified into three categories: 1) RNA with self-adjuvant characteristics, 2) components of the delivery system, and 3) exogenous immunostimulants. This review summarizes the three types of adjuvants used in mRNA vaccines and provides a comprehensive understanding of molecular mechanisms by which adjuvants exert their functions in mRNA vaccines.
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