There is such a thing. Before cowpox was discovered to prevent smallpox, the less virulent strain of smallpox was inoculated to prevent the more virulent strain. Inoculation is actually an ancient practice. The word "vaccine" is based on the latin word for the cowpox virus. "Vacca" being latin for "cow". So a vaccine, as first conceived, is novel because it confers immunity without actually being a strain of the disease it prevents.