When it comes to disease and medicine, the two most valuable things for preserving life, is infrastructure (shelter), and logistics. Back in WW2, troops that ended up in places far from urban hubs tended to be exposed to the elements or had to string out their supplies. As such, they tended to die from a mixture of cold, mosquitos, heat, famine, wet, and filth. Where America is concerned, it SHOULD be relatively safe for most soldiers, because there is an abundance of roads that pass by some form of shelter.
However, I suspect that the Trump Regime would have difficulty with managing logistics. It seems like that they don't value nor understand supply chains or other fine details. The National Guard troops that were deployed to LA, for example, didn't have bedding for resting. While those troops can tank an one-off like that, the Regime has been dismantling all sorts of things that keep people in tip-top shape. For example, the erosion of the weather service, means that soldiers will be more easily exposed to bad conditions. The destruction of food quality inspection, means that troops are more likely to have bad rations. Medical services are being rolled back, too. Trade with foreign nations is also likely to be cut off or reduced, which also means that troops could be lacking boots. (Quality boots are literal life or death for deployed troops.) All of this adds up to troops becoming vulnerable to the Four Horsemen.
It is my expectation that the Blue States would win a prolonged civil war, simply because they tend to understand the implications of policy.
While not directly related to vaccines, the latest episode of Perun goes over the economics of soldiers, including medical costs. It is a good channel if you want analysis about modern conflicts. I wouldn't be surprised if Perun starts covering America itself in the coming decade.
The Long-Term Costs of War - The Price of Life, Economics of Casualties & Russia's War