Let's play a bit of "What If" shall we?
Suppose some hostile country mounted a biological warfare action against us. Say they had developed a fast-spreading disease with a 3-5% mortality rate among senior management types. Let's say they started spreading it using widely traveled vectors in the international business community who would be the most likely to come in contact with the senior management types they wanted to target.
Here's what a caseload graphic might look like:
It seems that it might be 2-4 months before anyone who mattered actually noticed, and about a year before the maximum effects would hit. At this point, you should have your pre-vaccinated troops ready to rectify whatever borders you want, facing a demoralized enemy with its leadership significantly reduced. If your troops can offer a vaccine, however unproven, to your new citizens, they might even be happily welcomed, especially if the actual source of the disease can be adequately hidden. The NYT produced a graphic suggesting that a Covid vaccine might take as long as 10 years to produce in quantity, assuming standard super cautious methodology favored by the FDA.
A biological war would therefore, be very slow at the start, be very confusing in the middle, and fairly quickly done with at the end.