Schematic overview and time course of vaccine study. GPCMV seronegative outbred Hartley guinea pigs were immunized twice (interval of 3 weeks between doses), with vAM409 vaccine, or placebo. Small-volume bleeds were performed by toenail clip were obtained (arrowheads) on all guinea pigs following vaccination at weeks 2, 3, 5, 7, 9 and 11, for both quantitative PCR analysis of viral load and for analysis of antibody response. Challenge with SG-passaged GPCMV was administered on days 89–91 of the experiment, approximately 7–8 weeks after initiation of breeding.