Current vaccines based on production of neutralizing antibodies fail to prevent the infection and transmission of SARS-CoV-2 Omicron and its subvariants. Understanding the critical factors and avoiding the disadvantages of vaccine strategies are essential for developing a safe and effective COVID-19 vaccine, which would include a more effective and durable cellular response, minimal effects of viral mutations, rapid production against emerging variants, and good safety.

The development of human respiratory syncytial virus (HRSV) vaccine was hampered by an enhanced respiratory disease due to Th2-biased immune response. In this study, adjuvants MA103 and aluminum phosphate (Adju-Phos) were used to verify the immunogenicity of RBF protein (F protein expressed by E. coli). Both adjuvants significantly increased the neutralizing antibody titer and the number of IFN-γ-secreting CD4+ T cells in mice. However, from the IgG1/IgG2a and IFN-γ/IL-4-secreting CD4+ T cells ratio, MA103 significantly enhanced Th1-biased immune response. The pathological damage of lung in group RBF/MA103 was slighter than that in group RBF/Adju-Phos. In addition, the HRSV copies in the lung of group RBF/MA103 decreased by approximately 3×log10. These results suggested that MA103 provided better protection in mice.