As with any vaccine for viruses, there are many factors that may or may not influcnce the effectiveness of a vaccine. There are enveloped viruses and non-enveloped viruses, there are RNA viruses and DNA viruses, some viruses can have an almost immediate effect such as the flu virus and there are others like the slow viruses which can take 20 years or more to cause disease, these as well as whole of other factors can add significant challenges in making an effective and safe vaccine. Some commentators will use the failures or complications of past vaccines in support of their anti-vaccination point of view. Bottom line - Although it is not possible to prove a negative effect, I think I am safe in saying: Vaccinations have prevented more deaths than any other protective measure including the use of seat belts.

Pete