Full disclosure; no biology since 9th grade, so please go easy on me!
Look! It's a statue of that horrible animal that trampled Steve!
and the soldiers decide to destroy and dispose of it rather than to let it in.
The title of this XKCD is
Viral Vector Immunity
with the subtitle
How Vaccine Failure Due to Viral Vector Immunity Works
Wikipedia's Viral_vector; Vaccines explains the basic idea, which is (I think) that the viral vector's genome is modified so that it includes a new protein on its surface that serves as an antigen, and the host builds and learns an immune response to this antigen that will then immunize the host against an infection by another organism that also presents this antigen.
Question: The XKCD shows recognition and an analogy of an immune response, so why does the subtitle call this a "Vaccine Failure"?