Typically, historical novels are set before the 19th century and into the early 20th century. Historical medical questions can be some of the more difficult ones to answer because it’s challenging to find source material from the time.
However, when it comes to medicine, historical might be considered a time frame of more than ten to twenty years ago because of the rapidly evolving nature of the practice of medicine. One example of this would be CPR guidelines. Did you know CPR guidelines generally change every five years? To put it simply, the way we are doing CPR now is not the way it looked even ten years ago. Often times, what a writer might consider a contemporary medical question is truly a historical one.
I came across this resource called The History of Vaccines which reviews what vaccines were available when.
If you’re curious whether or not a character could have had the potential to be vaccinated against a certain disease, this source would be great to check out.