Strictly speaking, you don't need to “inject” them anywhere at all. There are tons of bacteria living on human skin, and one of them turned out to be quite interesting. Guys called Staphylococcus epidermidis live on every person's skin, and they are not doing anything particularly bad. But they have a peculiarity: in the course of their life, these bacteria can elicit a strong immune response from the host organism. Scientists have modified the bacteria so that they carry the tetanus gene, and it worked as a tetanus vaccine. But only in mice. Further research is needed to see if the same method works in humans. In the future, vaccines may become more accessible: you will need to rub a conditional ointment with bacteria into yourself. No shots, no sharp injections, no waiting in lines at hospitals, or anything else