So it is partially effective it seems. Since it is not an exact match but still a partial match, the mutated strain may still be identified by your immune system as a ‘partial match’ and reduce how long your are sick once the mutated virus reaches your lymph nodes a day after. Then the memory T cells in your lymph nodes generated by the vaccine may help identify the mutated strain and how to destroy it.

I think the big improvement in vaccines will be to create memory T Cells that can identify multiple viruses and mutations. That could lead to a universal flu vaccine.