2003 May 19, hansaplatz, “is this where the severitus challenge came from?”, in alt.fan.harry-potter[1] (Usenet):
it makes me wonder if the severitus thing could end up true but that wouldbe soooo ironic
2007 September 5, A. G. McDowell, “Re: The Future of Fan Fiction”, in alt.fan.harry-potter[2] (Usenet):
Arguably Snape's lifelong obsession with Lily in canon is more disturbing than the least imaginative and most derogatory Severitus conception, the drunken one night stand.
2011, Suzanne Passmore, "Rereading the Magical World of Harry Potter: Fanfiction as an Alternative Critical Modality", thesis submitted to the University of Western Australia, page 52:
Unlike the other Severitus fanfics analysed in this chapter, there is not a simple progression from dislike to affection and family feeling.
2015, A. J. Koning, "The Famous and Their Followers: Fanfiction as an Alternative Form of Literary Criticism", thesis submitted to Utrecht University, page 22:
[…] and in making Lily break her vows to James, Severitus fanfiction authors undermine the status of the heteronormative family in the original series.
2015, Agnieszka Oberc, "I Write. You Write. They Write.: The Literary Works of Fandom as a Factor in Integrating the Community", in On-line/Off-line: Between Text and Experience Writing as a Lifestyle (eds. Jaroslaw Pluciennik & Peter Gärdenfors), page 72:
The challenge became so popular that all stories in which Snape is Harry’s father are commonly called “severitus”, even if they do not meet the other criteria of the original challenge.
2019, Alena Pospíšilová, "Metamorphosis of Relationships in Harry Potter Fanfiction", thesis submitted to the University of Pardubice, page 63:
The story falls into a fandom subgenre called Severitus, which features a very uncommon kind of relationship to be found between Harry Potter and Severus Snape - one of a son and a father.
