Doubtful. Insurances dont run at a loss, every penny that goes into them they take a cut as profits. If the treatment is expensive, great! More demand for their product, and even if their cut is the same, their profit and revenue will be greater.
There's an 80/20 rule in the ACA [0] where at least 80 percent of the premiums paid must be spent on healthcare costs, otherwise the difference between 80 percent and the actual amount paid must be returned.
Sounds great, until the insurance companies figured out that the best way to make more profit with this profit cap is to pay out more and increase the premiums accordingly. [1] Obviously, the hospitals don't mind that either.
I'm sure they would, but bear in mind that the costs are likely going to be much higher for an MS vaccine than for nCov2, due to the economies of scale that the latter has. Insurance companies will still probably love a one-time $20,000 (or whatever number they can justify) treatment compared to ongoing costs.