burial-ground
Explanation
The Joke
The comic plays on the classic haunted-house horror trope where a house is built on an ancient burial ground. A person announces: "I''m afraid this house is haunted. It was built hundreds of years ago on the site of a twenty-first century burial ground." This immediately flips the usual framing -- instead of the burial ground being ancient and mysterious, it''s from our era (the 21st century), which from the perspective of the characters hundreds of years in the future is the "ancient" period.
The ghosts then appear, and instead of being menacing specters, they are "goddamn anti-vaxxers trying to make us ill again." The people in the house react with exasperation rather than fear, and someone suggests they just sell the house at a loss and move on. The haunting is not supernatural terror but the annoyance of dealing with the irrational beliefs of people from our time period.
The Humor
The joke works by combining two comedic reversals. First, it subverts the haunted-house genre by making the "ancient burial ground" be from our own era -- reminding us that someday the 21st century will be the distant past that future people look back on with a mix of curiosity and horror. Second, it replaces the typical ghostly menace with something far more annoying and topical: anti-vaxxers. The ghosts from our era are not scary because of supernatural powers but because they represent the spread of dangerous misinformation.
The comedy also lies in the idea that of all the things future generations might remember about the early 21st century, the anti-vaccination movement would be the defining and most irritating legacy -- bad enough to haunt people from beyond the grave.
References
The "house built on an ancient burial ground" is a classic horror trope, most famously associated with the film Poltergeist (1982). The anti-vaccination movement, which gained significant traction in the early 21st century partly due to the discredited 1998 paper by Andrew Wakefield linking the MMR vaccine to autism, has been a frequent target of SMBC''s scientific humor.