Vaccination & epidemic

[xoxoxoBruce]

these viruses live and divide and grow in lymphocytes -- the immune response cells, the B and the T cells. So when you give a vaccine, you send your B and T cells in your immune system into overdrive. That's its job. Well, if you are harboring one virus, and you replicate it a whole bunch, you've now broken the balance between the immune response and the virus.

So is it a matter of timing, introducing the vaccine before the virus has been defeated?
If the virus is never defeated, and it's a never ending battle, then wouldn't any infection, anything the immune system is called on to battle, cause the virus to spread?