I'm very obviously not a lawyer nor lawmaker, but I'm under the impression that anything can be classified as an act of war as long as another country is willing to treat it as such, and other superpowers/blocs are willing to sign off on this assessment, and back it militarily and through sanctions/incentives.
(Though this would all probably swept aside as soon as somebody reverse engineers the vaccine.)
(Though this would all probably swept aside as soon as somebody reverse engineers the vaccine.)