Heterologous sequential immunization using chimeric mRNA and protein vaccines with HA-stem and S-RBD enhanced protective mucosal immunity against influenza and COVID-19.

Influenza virus and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), as two highly contagious respiratory pathogens, continue to pose critical challenges to global public health. In our previous research, we successfully developed mRNA and protein vaccines by integrating the HA stem of H1N1 influenza virus with the RBD from SARS-CoV-2, which demonstrated broad protective efficacy against multiple strains of both viruses in mouse models. Here, we compared the immunogenicity and protective efficacy of heterologous sequential immunization regimens based on this chimeric antigen design, employing mRNA vaccine priming followed by intranasal protein vaccine boosting. Our results show that an mRNA vaccine prime followed by an intranasally administered protein vaccine with PICKCa adjuvant boost not only induces robust systemic humoral immunity but also significantly enhances respiratory mucosal immunity. Compared to boosting with an adjuvant-free intranasal protein vaccine, the inclusion of PICKCa adjuvant markedly elevated mucosal IgA levels in both nasal washes and bronchoalveolar lavage fluid (BALF). Furthermore, the adjuvanted intranasal protein vaccine boost regimen provided optimal protection against high-dose lethal influenza virus challenge. These findings provide valuable insights for refining immunization strategies to enhance the efficacy of combined influenza-COVID-19 vaccines.