Viral mutation rates play a pivotal role in vaccine development. Mutation rate refers to how often genetic changes occur in a virus’s genome during replication. All vaccines trigger immunity, but how long it lasts depends on several factors. One of the important ones is the rate at which a virus replicates. If a virus replicates quickly, it has a chance to produce more mutations, also known as variants. The more variants emerge, the harder it is to make a vaccine that will create lasting immunity, because the target keeps moving. If a virus is stable, that gives us a big advantage. Measles is an example of a stable virus that is unlikely to replicate, so scientists could predict that immunity would last a long time, which it does." Smallpox and polio, highly contagious viruses that were almost eradicated through vaccination, are also stable with low mutation rates.