I was a bit confused by the ending, so I read the comments under the Chinese translation, and I found some neat theories. One person suggested that the monsters might be a metaphor for homophobia in Japanese society, which is possible, but I think it's a bit far fetched. I think the explanation that makes the most sense is that it's a story about drug abuse - we know the new flu is real based on the newspaper at the end, but maybe the pills the girls took had hallucinative effects, which is why they thought the monsters existed. Could even be that they were taking the actual medicine for the flu, but took much more than is considered safe. The author also made a so-called "remake" of this manga last year (https://x.com/korusee/status/1855943141330911439), which is also twice as long, and I haven't found it available anywhere digitally, but the preview already shows some kind of medicine as a central theme, so it's definitely somehow important. Edit: the monsters knocking on the door might've also been concerned adults, since in the end they are just kids who locked themselves up in a room for I'm assuming quite a while. Also explains how their bodies were found.
I think it's a combination of the two theories. The two didn't have the support systems they needed so they already didn't like the people around them. The drugs then could have exasperated their perception of the people around them into monsters based on their pre-existing experience.
Anyways, absolutely insane read I was not expecting to see today. 10/10