You're only going to see this in the case of live attenuated vaccines. They can mutate and become virulent (since they were mutated to attenuate them in the fi

You're only going to see this in the case of live attenuated vaccines. They can mutate and become virulent (since they were mutated to attenuate them in the first place). You're not going to see that with mRNA vaccines, you're not going to see that with the adenovirus-vectored vaccines. You're not going to see that with inactivated virus vaccines (like most influenza[*] or CoronaVac or the Sinopharm COVID vaccines). Live attenuated virus vaccines are one of the oldest vaccine methods and have been around since the 19th century. They're created by culturing the virus serially in artificial media and isolating a strain which is less virulent. Very brute force laboratory methods that only require the ability to culture the virus and an animal model of virulence with decent correspondence to human virulence. One thing to remember is that we're dealing with a medical triage situation and its better to cause a very small amount of disease in order to eradicate most of it. People get hung up on the fact that medical science is a human activity that doesn't have perfectly flawless outcomes like in Star Trek, while the natural diseases are orders of magnitude worse. [*] Except the Flumist nasal spray is live attenuated. Still with influenza that isn't likely a big deal. I would be willing to wager a bit that this technique is still being used there because the influenza virus is very good at getting past the nasal mucosa and we don't know how to engineer particles that do that very well (still no nasal COVID vaccine).