BACKGROUND: Human monkeypox cases are escalating worldwide. Smallpox vaccination, compulsory in Austria until 1981, was reported to confer 85% cross-protection against monkeypox.

METHODS: To assess the impact of smallpox vaccine-induced protection, the age-dependent vaccine-induced immunity against human monkeypox and the probability of infection according to age in the general population of Vienna, Austria, were determined employing a modified Susceptible-Infected-Removed model.

RESULTS: In the population born before 1981, the average vaccine-induced protective effect was calculated at 50.4%, whereas in the population born thereafter protection is lacking. The overall probability of infection after exposure to an infected patient was calculated at 73.8%, which exceeds the threshold value of 46.9% for an index patient to infect at least one other person (R≥1.0).

CONCLUSION: Our modelling shows that, if no additional interventions are taken, the collective immunization status of the population alone will not suffice to contain human monkeypox. Although the majority of cases have occurred in a subpopulation, given the steadily increasing incidence, dissemination into the general population remains possible, as observed before with HIV. Our model emphasizes the need for adequate containment measures and may aid in specific risk assessment, as it can easily be adapted to other populations and cohorts worldwide.