Listening to other scientists, the vaccine is tailored specifically to a certain strain. Also, similar strains that don't differ much can also be defeated. However, the virus eventually mutates to a point where it easily defeats the anti-body response from the vaccine. Normally this isn't a problem, but when you engage in mass vaccination with a virus that's very active, the large amount of people vaccinated essentially acts as a petri dish where the virus can easily jump hosts and rapidly learns to defeat the vaccine. Furthermore, natural protections are diminished when the vaccine now releases useless anti-bodies, so essentially the immune system rolls out a red carpet to the virus to enter the host's cells.