In science fiction, we see all kinds of nightmare viruses that evolve immunity to vaccinations. The scary part? It could actually happen in real life, especially if anti-vaxxers encourage people not to get vaccinated. Populations with pockets of non-vaccinators are more likely to evolve vaccine-resistant viruses than populations where everybody gets a vaccination. ...
Keeping a group up-to-date on their vaccinations — especially children, who haven't been exposed to many types of the flu — will reduce the chance that the virus will be handed back and forth between the exposed and the naive populations. Keeping vaccination rates high can stop us from needing new vaccinations.