The potential global risk of monkeypox importation

In a recent study posted to the medRxiv* server, a team of researchers hypothesized various scenarios of travel restrictions to estimate the potential global risk of monkeypox virus (MPXV) importation.

Study: Impact of airline travel network on the global importation risk of monkeypox, 2022. Image Credit: Song_about_summer/Shutterstock

Background

The World Health Organization (WHO) declared MPXV as a global public health emergency on July 23, 2022. Earlier, it was not considered contagious before symptom onset; however, several case reports have identified asymptomatic MPXV infections raising concerns of a multi-country outbreak. It is more worrisome that the average incubation time of MPXV is barely 8.5 days, in some cases, up to 21 days, but that still makes its incidental importation easy.

Furthermore, the global cessation of smallpox vaccination in the 1980s, which was considered effective against MPXV as well now increases susceptibility to MPXV. All countries are at high risk of MPXV importation because of the global travel network. The 2022 MPXV outbreak began in the United Kingdom (UK) due to MPXV importation from Nigeria.

As of September 4, 2022, the United States (US), Brazil, the UK, Spain, and France have the highest aggregate number of MPXV cases. Although it is not a sexually-transmitted disease, it has uniquely spread in these countries via men who have sex with men (MSM). Now that airline travel is returning to pre-coronavirus disease 2019 (COVID-19) levels, International Air Transport Association (IATA) expects air passenger volume to be 69% more than in 2019. It is possible that these travelers could unintentionally cross the border with MPXV infection.

Previously, researchers had used mathematical modeling to simulate airline transportation networks arriving from Wuhan, China, from where the COVID-19 pandemic began, and quantify the impact of travel restrictions against the risk of importation.

About the study

In the present study, researchers used a mathematical model similar to the one used during the COVID-19 pandemic for estimating the importation risk of MPXV. They used public data sources to gather passenger volume data from 1680 airports in 176 countries that reported confirmed MPXV cases.

The researchers estimated passenger volume (PV) on an airplane based on the maximum number of seats in that plane. Further, they used the airline network to determine effective distance, i.e., the minimum distance among all possible paths accounting for the PV-weighted path length and the degree of each node corresponding to one airport.

Notably, the effective distance worked on the assumption that MPXV began spreading from the UK. However, the team also performed sensitivity analyses using a closeness centrality index on the airline network to assess the attractiveness of an airport in the model from the PV point of view instead of the effective distance.

Furthermore, the researchers estimated the importation risk based on the patterns of domestic and global population movement using a survival analysis technique in which the hazard function was a function of effective distance. They multiplied the PV by 0.93 to estimate domestic travel and 0.69 for international travel.

For studying travel restrictions, the researchers selected those scenarios which reduced the passenger volume from/to countries with detected MPXV cases. Likewise, they focused on those scenarios that increased or decreased the global passenger volume relative to high and low volumes observed in 2019 and 2021, respectively, irrespective of MPXV cases.

Study findings         

At similar flight volumes as 2022, the risk of MPXV importation was expected to be substantial in multiple locations by 31 December 2022. Regardless of the distance from London, UK, locations with large PV and closely connected flights were at higher risk of MPXV importation.

The reduction of PV by 50% or 90% from or to countries already having MPXV importation did not modify the MPXV importation risk to a great extent. The degree of PV had a nonlinear effect on the MPXV importation risk reduction because the connectivity between each airport network also influenced the optimal size of volume reduction. PV reduction to minimize the risk of MPXV importation might not be an efficient strategy for making practical recommendations regarding travel restrictions.

Conclusions

The study model could not effectively capture the complex dynamics of global disease transmission. Yet, it showed that airline travel restrictions in most areas might not prevent MPXV importation in real-world settings. The study results highlighted the importance of enhancing the surveillance against MPVX in countries with a high risk of importing this virus. In addition, these countries at high risk of MPXV importation should focus on contact tracing and isolation techniques.

Furthermore, the study findings on travel patterns could inform public health interventions, especially for understanding the risk of observing an emerging disease across the border.

*Important notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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