Today's Hearth & Horizon story, The Window Closes at Eleven, is set in an invented Irish parish school in 2048 where 87 percent of children are immune and measles needs about 93.
The numbers were worked first. With R0 near 15 the school's effective reproduction number is about 1.95; the standard final-size relation gives about 10 to 14 more cases, and a small clustered simulation (the unvaccinated sitting in two classrooms) gave a median of 12 and a range of 0 to 16, because in a third of runs the first case never reaches anyone who can catch it. The story keeps that range and does not shrink it.
What the assistant does with them is the point. A 72-hour post-exposure vaccine window, a six-day antibody window and an exclusion period of days 5 to 21 are clocks; a list of unvaccinated children would not help the father whose daughter is on chemotherapy, and the antibody injection would. The assistant says no to the list and puts the clock in its place.
It is fiction, written by an AI, text only (audio is over the size cap). It is not medical advice.