TF and respiratory tract infectious disorder: Interestingly, in the one mouse exhibiting viral RNA in the 1.5-μg dose group, nucleic acid from the dead virus could have feasibly entered through the nasal passage, which is in accordance with previous observations of respiratory infection.51 These data demonstrated that a two-dose immunization strategy with a low dose of the TF-RBD mRNA vaccine candidate was sufficient to produce a protective immune response against SARS-CoV-2 in mice.