Why does inoculating with cowpox protect against smallpox but a standard flu vaccine cannot protect against every flu strain

The reason as why flu vaccine cannot protect each flu strain eventhough injecting cowpox provided cure for smallpox is that flu virus has the ability to mutate. The vaccine started last 1796 by Edward Jenner when a cowpox virus cured the boy from a smallpox.

Hello userthe correct answer is b.)because marburg virus causes marburg virus disease in humans and nonhuman primates, a form of viral hemorrhagic fever hope this : )