Congratulations to Victoria Romeo-Aznar (former postdoc, now in her faculty position in Buenos Aires) and to our co-authors from Brazil, for this paper. Here, we discover an intriguing pattern in the size of successive epidemic waves of dengue in the megacity of Rio de Janeiro and how this ratio behaves with population density, using data on reported cases at very fine spatial resolution. Why does this ratio matter? Because it is closely related to the dynamics of herd immunity and they interact with seasonality (that is, to the fundamental nonlinear dynamics of a seasonal infectious disease). Why do the findings matter? Because they show a clear role of fine-scale population density in these transmission dynamics that nevertheless provides an indication of how we can aggregate space and address spatio-temporal epidemiological dynamics in a complex urban landscape. (Apologies for a repeated sentence in the abstract — this typo is being corrected…).