Yes, it matters who is spreading monkeypox - Authors' reply.

Concerning the current monkeypox outbreak, we argued that it is irrelevant whether monkeypox mainly spreads in the community of men who have sex with men or whether it spreads in the general population; reducing its reproduction number to less than 1 to control the outbreak is all that is necessary. On the basis of well known results, Jonathan Smith points out the importance of transmission heterogeneity, but ignores the fact that our argument is independent of transmission heterogeneity. As stated in Lloyd-Smith and colleagues, on which Jonathan Smith's Correspondence is based, the central role of R 0 in epidemic analysis is unassailable: the epidemic dies out with certainty if the effective reproduction number R is lower than 1, yet the extinction probability is lower than 1 if R is higher than 1. Transmission heterogeneity increases the extinction probability of an outbreak caused by a single case, but this is no longer of concern in the current outbreak. The panel B of Smith's figure wrongly gives the impression that monkeypox outbreaks of 100 cases or more should be highly unlikely. In any case, these results are irrelevant for the outbreak with currently more than 23 000 confirmed cases. Transmission heterogeneity renders contact tracing more efficient for most cases (who caused few secondary cases), but also much more difficult for a few superspreaders; what eventually counts is the detected overall percentage of secondary cases.

Evidence of accelerated evolution of the virus and human adaptation suggests that R 0 might be larger than in earlier outbreaks. This finding might also be caused by a specific contact behaviour or routes of transmission. Because more than 95% of the cases are in the community of men who have sex with men and more than 90% are associated with sexual contacts, disease transmission during the early phase, during which no characteristic symptoms are visible, seems plausible. The fact that the virus has not found its way into other groups of the population on a large scale gives evidence that infection control works, in line with our original Comment. Although the number of cases seems to slowly decline (except in the Americas), the continued spread in the community of men who have sex with men suggests that contact tracing might still be difficult for the presymptomatic period in this group and that, therefore, a higher vaccination level is needed in this community. The high percentage of individuals who are HIV positive (and hence immunosuppressed) is alarming. In the context of increasing evidence of asymptomatic infections, which lead to silent transmission, more rigid contact tracing and routine tests in risk groups might be necessary in addition to our original suggestions of pre-exposure and ring vaccination.

We declare no competing interests.