Dirty mice better recapitulate key features of mRNA vaccine immunogenicity observed in humans.

Although specific pathogen free (SPF) mice have traditionally been used to test candidate vaccines, recent work has demonstrated that "dirty" mice with broad microbial exposure more appropriately recapitulate human immune responses. Using a model where lab mice are co-housed with pet store mice, we modeled SARS-CoV-2 mRNA vaccine responses in dirty and traditional SPF models. We found that dirty mice show reduced serum spike-binding antibody titers after prime and require a second booster dose to reach SPF-level spike antibody titers. Additionally, spike antibodies showed faster waning in dirty mice through 5 months post-vaccination, and neutralizing activity of these antibodies were reduced against Omicron variants, directly comparable with observations in humans. We further investigated the seasonality and consistency of pathogens in cohoused mice, and the impact of serial microbial exposure on our animal model system. We found that pathogen exposure and T cell activation remained consistent over time, and that a single co-housing event was sufficient to provide broad microbial exposure. This work demonstrates that the dirty mouse co-housing system is a promising, translationally representative approach to screen candidate mRNA vaccines for efficacy and durability prior to human clinical trials.