What the vaccine does is that it mimics an infection causing the body's immune system to react and fight against the virus, the virus being weakened. With the help of adjuvants such as aluminum, antibiotics and preservatives, the virus cannot become fatal or contaminated. It will not be able to develop bacteria thanks to the thimerosal preservative. The simulated infection doesn't actually infect you, it prepares the immune system in case you are infected with the real virus. Antibiotics also help fight bacteria that the virus might cause. Considering the contents of vaccines and the process, people should be careful and well-informed about vaccines administered by healthcare professionals. Even though we've come a long way since the Chinese were blowing scabs up their noses, vaccines are still far from being completely safe and effective.