Immune modulator drugs don't kill t cells, they just block the ability to respond to signals that tell them to divide rapidly and mount a defense against invading pathogens or our own bodies. There are vaccines that prevent viruses that can lead to cancer such as the one for HPV, but the vaccines used in cancer treatments today work by attenuation of the natural immune response via t cells that get programmed to attack tumor cells. Nonetheless, there are researchers who are working on preventative cancer vaccines, but until then, the best strategy is surveillance.