Help with understanding XKCD's “How Vaccine Failure due to Viral Vector Immunity Works” Why is this response a failure exactly?
Yes, the cartoon is a description of immune action.
Traditionally shown is first exposure being a vaccine, and the second exposure being the bad virus being destroyed.
However, this cartoon is describing viral vector immunity.
The first exposure is a viral vaccine. This alerts and educates the immune system about not just the intended antigen, but also the (harmless) carrier virus which is represented by a horse.
The second exposure is a different viral vaccine.
The immune system immediately mounts a successful response against the vaccine because is recognises the carrier virus.
So the immune system doesn't get the chance to learn about the antigen associated with the second virus.
A example of viral vaccine failure would be useful - I will see if I can find one if no-one else does.