The mRNA-based vaccine technology is gradually developed as one of the new vaccine technologies in recent years. The technology is becoming more mature in terms of its stability and efficient delivery. Antigens encoded by mRNA vaccines are expressed in the cytoplasm and can induce the activation of both B cell responses and T cell cytotoxicity against infectious pathogens. In addition, the simple production process of synthetic mRNA vaccines can facilitate rapid response to emerging infectious diseases such as 2019-nCoV infection. In order to understand the current status of mRNA vaccine research and development for infectious diseases as well as providing reference for the development of clinically applicable mRNA vaccine against 2019-nCoV pandemic, this paper mainly reviewed the structural characteristics, advantages and challenges of mRNA vaccines and the current situation of vaccine research application in infectious diseases.

Sequential immunization is one of the special means to solve the shortage of vaccines, respond to SARS-CoV-2 variants and improve the efficacy of vaccines in the current pandemic period. This article mainly reviewed five sequential immunization strategies using the vaccines authorized by World Health Organization: priming with inactivated vaccine and boosting with recombinant protein vaccine, vector vaccine or mRNA vaccine; priming with vector vaccine and boosting with mRNA vaccine; prime-boost immunization with mRNA vaccines produced by different manufactures. Results of the related studies showed that heterologous sequential immunization strategies were safe and effective, and higher immunogenicity and efficacy could be achieved by sequential immunization. In addition, sequential immunization could provide certain protective effects against SARS-CoV-2 variants.