If vaccines are made from viruses, why do they not make us sick?

Much research: Vaccines will often take 2 decades of research before they come to market. In that interval, the researchers determine whether a weakened/ killed or synthetic material that replicates parts of the virus will trigger a protective immune response. The weakened virus (nasal flu,MMR, varicella) all have some potential to cause a low grade infection. Killed or synthetic material does not.