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.
Edit 2: Actually the homophobia theory is also starting to make more sense for me. If you assume that the flu itself is a metaphor for homosexuality, and the people taking the "cure" for it, aka the monsters, are homophobic, then it makes sense why they'd be depressed enough that they'd do anything to not turn out the same and lose their true selves.