I was being a bit imprecise when using the phrase "strain." In the context of virology in general, a strain is delineated by a difference in functionality - i.e. some key component of how it infects, replicated, or damages the body is different from other known cases. However, in the context of vaccines, a "strain" may function the same but might not produce some unique protein or other marker that the immune system uses to identify the pathogen, without having an effect on the rest of the virus.

COVID vaccine work is still in its infancy so we don't know yet whether the non-functional mutations we're seeing in COVID's phylogeny will effect how effective vaccines are.