This invention relates to methods of removing bioburden from an aggregate of cells to obtain sterile cells that remain viable and immunogenic for the production of vaccines. This invention further relates to a method of eliciting an immune response to prevent a recurrence of metastases that involves preparing and administering a sterile vaccine derived from solid tumors. The vaccine is prepared by excising a solid tumor from a cancer patient, digesting the tumor cells with an enzyme to obtain dissociated cells, irradiating the dissociated cells to render the cells non-tumorigenic, and sterilizing the cells.