Disparities in transmission dynamics of the 2022 mpox outbreaks between Europe and Americas

Dear Editor,

The 2022 mpox (monkeypox) outbreak was first sparked in Europe, and subsequently spread to Americas. There are several unique features of the 2022 outbreaks in non-endemic countries. Men who have sex with men (MSM) are the main affected population, and the transmission is primarily driven by sex-related intimate contacts. However, there appears substantial disparities in the dynamics of the epidemic evolvement between Europe and Americas.

The reproduction number (R) is a hallmark of epidemic transmission dynamics. We estimated the time-varying reproduction number R_t for the 2022 mpox outbreaks in Europe and Americas, respectively. At the early stage, the R_t value was higher in Europe compared to that in Americas. However, since 27 June 2022, the R_t value in Americas rapidly surpassed that in Europe, which corresponded to the rapid escalation of reported cases, particularly in the USA. Although the reproduction numbers gradually declined in both regions, there remains a substantial gap (Fig. 1A). For example, the average reproduction number in Americas is 0.82 (95% CI 0.76–0.88), while that of Europe is only 0.62 (95% CI 0.44–0.84) during December 2022. This is in line with the confirmed cases of about 2000 and 160 in Americas and Europe in that month, respectively [1,2].

Fig. 1.

Disparities of the mpox transmission dynamics in Europe and Americas.

A. The time-varying reproduction number estimated for the 2022 mpox outbreaks in Europe and Americas by using the R package EpiEstim. The mean serial interval was assumed as 9.8 days and standard deviation as 8.5 referred from the UK Health Security Agency (Investigation into monkeypox outbreak in England: technical: briefing 1, 2022). The timeframe ranged from 01 June to 31 December 2022. Data of reported cases were retrieved from https://worldhealthorg.shinyapps.io. B. The age distribution of mpox cases reported in Americas and Europe. C. The sexual orientation of mpox cases in these two regions. Data of age distribution and sexual orientation were obtained from https://worldhealthorg.shinyapps.io/mpx_global/#3_Detailed_case_data.

We postulate that there are several possible factors contributing to these disparities in transmission dynamics, including the levels of awareness, demographic characteristics and countermeasure implementation. As shown by the age pyramid, a larger proportion of reported cases in Americas compared to Europe (73.4% versus 61.3%) are in the group of under 40 years old (Fig. 1B). This young group of infected patients with high sexual activity may sustain the transmission. Secondly, sexual orientation of the majority (96.0%) of European cases is MSM, whereas this proportion in Americas is 77.9% (Fig. 1C). This suggests that the scale of spillover to other populations is likely larger in Americas. This may also explain that, despite a 50% reduction of sex partners or one-time sexual encounters in the USA [3], the outbreak remains not fully controlled. The awareness of mpox is presumably much lower in the general compared to high-risk populations, and transmission in general community would pose additional challenges in containing the outbreak.

The smallpox vaccines are currently being used for preventing mpox spread through ring vaccination or vaccinating high-risk populations. Although it is assumed to have 85% efficacy in cross-protection against mpox [4], there are emerging concerns regarding the actual efficacy which requires further assessment in particular in at-risk populations [5]. Furthermore, the accessibility to the vaccines varies tremendously even within the American region. The US government has prioritized to vaccinate 1.7 million people who are considered at highest risk, whereas the accessibility to these vaccines is very limited in Latin America.

Encouragingly, the reproduction number has gradually declined and is currently (January 2023) below one in Americas. However, there remains several contemporary challenges in containing the outbreak. Because of the heterogeneous capacity across different countries, Americas need to move more cohesively and quickly in responding to this crisis.

Conflict of interest disclosures

None reported.

Handling Editor: Patricia Schlagenhauf