Even if it did escape from one of those biological research centers, it's unlikely that another vaccination programme couldn't beat it. After all, the reason the vaccines was so successful was that it was inexpensive, you only needed one dose per person, very stable and effective nearly 100% of the time, because Smallpox doesn't mutate very fast. The whole problem with infectious diseases these days is a double-edged blade, bacterial diseases are becoming increasingly difficult to thwart with resistances to most antibiotics popping up, and some species just mutate too fast to make a lasting vaccine for them. (loving histocompatability proteins)