[privong]

> but would they be useful against such a rapidly-changing pathogen ?

Does that explain why people are not immune from the flu after having it once – the version they have antibodies to is not the same as the mutated version which circulates later?

[learc83]

>Does that explain why people are not immune from the flu after having it once

Yes. There are many strains of Influenza, immunity to one strain doesn't necessarily confer any immunity to another strain.

However, Ebola mutates much less rapidly that Influenza.

[dreamcompiler]

Yes. This is why you have to get a flu shot every year: The flu virus is different every year. The same rapid-mutation phenomenon happens with the so-called "common" cold: Having a cold once provides no immunity to prevent you from getting one again.