Minimal effective vaccine dose in two tumor prevention models. Groups of four or five mice were vaccinated once, twice or three times (1X, 2X, or 3X) at 2-week intervals with virus-like replicon particles (VRP)-neu or VRP-hemagglutinin (HA) (104 IU or 105 IU). Two weeks after the final vaccination, the mice were challenged with A2L2 cells injected into a mammary fat pad or intravensouly. The mice were killed 5 weeks after the tumor challenge. (a) The mass of the mammary tumors, if present, was determined in the mammary fat pad prevention model group, and (b) the number of surface metastases on the lungs was determined in the experimental lung metastasis prevention model group.