Duration of challenge immunity to coronavirus JHM in mice

The duration of challenge resistance in mice immunized with mouse hepatitis virus (MHV) strain JHM was examined as a model of immunity to corona-virus infection. Genetically susceptible BALB/cByJ mice were immunized by intranasal (i.n.) or per os (p.o.) inoculation with MHV-JHM or sterile tissue culture fluid (sham) then challenged i.n. with MHV-JHM or sterile tissue culture fluid 1, 6, or 12 months later. Four days after challenge, virus in nasal turbinates and liver was quantified, and prevalance of microscopic lesions in liver and gut-associated lymphoid tissue was tabulated as indices of challenge resistance. MHV-immunized and challenged groups were compared to sham-immunized and challenged groups. Mice immunized by i.n. inoculation were strongly resistant to challenge at 1, 6, and 12 months. Mice immunized by p.o. inoculation were resistant at 1 month, but became partially susceptible to reinfection at 6 and 12 months, based upon all indices. These data indicate that, depending upon route of immunization, mice can become susceptible to reinfection with the same coronavirus strain over time.