Does it matter who is spreading monkeypox?

Recently, many cases of monkeypox were reported worldwide. Although most of these cases seem to be associated with the community of men who have sex with men (MSM), not all of them are. Cases with west and central African monkeypox virus clades have been rising in the past 20 years. The current spread, due to the less transmissible and less virulent west African clade, was unexpected because observations on earlier outbreaks, mostly in African villages, indicated that monkeypox outbreaks are self-limiting. In technical terms, the basic reproduction number (R0) was less than 1. R0 is the average number of secondary cases produced by a single case during the whole infectious period in a community without immunity and without interventions. The low transmissibility, obviously, must have changed during this current emergence of cases. Whether this change was due to mutations in the virus or due to a different type and frequency of contacts is interesting, but not necessarily relevant when contemplating how to stop the outbreak. The main question simply is how to reduce the average number of secondary cases per infected person to below 1. In this context, it also does not matter whether the infection mainly spreads within an MSM community or finds its way into other groups of the population. We can only speculate about the current value of R0 for monkeypox.

Monkeypox used to be far less transmissible than smallpox, the R0 of which lay between 3·5 and 6. The R0 has not been estimated for the west African monkeypox clade, but the effective reproduction number (the average number of secondary cases per infectious case in a population made up of both susceptible and non-susceptible hosts) of a more transmissible clade in the Congo Basin was estimated to be about 0·3, while its R0 was estimated to be between 1·46 and 2·67. New estimates are urgently needed, particularly because these estimates date back to a time when smallpox vaccination coverage was high, which might have led to an underestimation of the R0. Using R0=3 for monkeypox might still be considered a very large (highly pessimistic) value. As with smallpox, individuals who are infected take rather long to develop symptoms and they have—compared with influenza or SARS-CoV-2—a rather long infectious period. Therefore, the average generation time of monkeypox is rather long (about 20 days). This renders monkeypox highly vulnerable to interventions: even with R0=3, it takes months until a few thousand cases occur. With the raised awareness, cases will be detected much quicker now than in the beginning of the outbreak. Even if it takes 1 week from onset of symptoms to detect and isolate cases, the contagious period (15–27 days after the onset of rash) is reduced by over 50%. If the infection is not predominantly passed on during the prodrome or early enanthem periods, the assumed reproduction number of 3 drops to less than 1·5. The standard practice of most public health systems is to initiate contact tracing for all confirmed cases. As the latent period of monkeypox is long (7–17 days), cases among known contacts can be contacted before they spread the infection. They can either be quarantined or, at least, they will be aware of the infection as soon as they develop first symptoms. Importantly, unlike for SARS-CoV-2, there seems to be no evidence of asymptomatic Orthopoxvirus infections in humans, or at least this is believed to be true for smallpox; studies for monkeypox are still ongoing. Because oropharyngeal lesions are necessary for airborne transmission and skin rash is required for transmission by physical contact, it is plausible that at least some symptoms must be present before the infection can be passed on. Therefore, awareness or quarantine of contacts should further reduce the spread of infection considerably, bringing the remaining reproduction number close to or below 1. Data from the 1980s suggest that smallpox vaccination provided 85% protection against monkeypox. In the 1970s, smallpox vaccination programmes were starting to be discontinued and, by 1984, all countries had ceased vaccinating the general public against smallpox. Studies suggest high seropositivity of Orthopoxvirus antibodies in the smallpox-vaccinated population. This further reduces the effective reproduction number of monkeypox. Using the newly available monkeypox vaccines, post-exposure vaccination of known contacts—or more generally the social environment of cases (this was termed ring vaccination)—can even protect people who have already been infected if applied soon after infection. Additionally, increased awareness might lead to earlier detection of new cases, and the prophylactic vaccination of risk groups and of individuals who are immunocompromised can also be considered.

We declare no competing interests.