Galen was at his peak during the Antonine Plague, and for the sake of discussion, let's assume that it was indeed smallpox that was the culprit for the plague. Galen had managed to figure out that there were tiny particles, or "Bad seeds", so small that they can not be seen, which carried infection (bacteria and viruses-that conclusion would not be acceptedagain until the 18th century), so I don't think it is outside the realm of possibility that he could create an inoculation for smallpox.
Could Galen have created a smallpox inoculation? If so, what would the effects be, keeping in mind this is in the middle of the Antonine Plague.