Why are newly mutated influenza viruses especially harmful to humans?

Explanation:

If they´re mutated it means that there was a modification in they´re genes, which makes them resistant to any actions that us humans took to prevent the previous generation of influenza viruses.

Since there was a change in their genetic code to resist our treatments/ preventions, they pose a higher risk to us. Why? Because our treatments/preventions will be useless and the virus might cause some damage until our solutions to prevent/cure it are updated, that is, if we don´t have to re-invent any.

Tip:

You can think of the influenza virus as a software. The mutation can be an update that gets rid of a bug, which would be our preventions (vaccines).

Hope it helped,

BioTeacher101