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Monkeypox: What do we know about the outbreaks in Europe and North America?

BMJ 2022; 377 doi: https://doi.org/10.1136/bmj.o1274 (Published 20 May 2022) Cite this as: BMJ 2022;377:o1274

Rapid Response:

Surveillance bias of monkeypox

Dear Editor

Are we on the verge of a monkeypox epidemic? If not of a pandemic? Several cases have been recently reported in non-endemic countries, with no direct links to endemic areas (1). The number of cases is on the rise, and it will likely continue to do so in the near future (2,3). Media coverage is high, and news spreads at an amazing speed. Health authorities are under pressure and need data to inform citizens. But how reliable is the trend in the number of cases? What is the real scale of this outbreak?

Monkeypox surveillance is based on data from healthcare providers. These data are event-based rather than population-based and are not designed primarily for surveillance activities. They can be exposed to surveillance bias, a bias that arises when differences in the frequency of a condition are due to changes in the modality or intensity of detection rather than changes in the actual risk of the condition (3). Actually, the more we search, the more cases we find. The intensification of surveillance, notably through contact tracing, will lead to the diagnosis of cases that might not have been identified in usual circumstances. The high media coverage, as well as dread spreading through social media, will encourage health-seeking behaviors and clinicians’ alertness, contributing to the increase in the number of cases identified.

As a consequence, it is difficult, at least for the moment, to assess the scale and the dynamics of the outbreaks. Moreover, comparisons between regions are limited due to differences in intensity of screening and diagnosis activities across healthcare systems. Surveillance data are key to breaking the chain of transmission and eventually helping contain monkeypox outbreaks. However, we need time and careful analyses accounting for surveillance bias to make sense of these data for sound decision-making.

Stefano Tancredi, MD, fellow in public health, and Arnaud Chiolero, MD PhD, epidemiologist and professor of public health, from the Population Health Laboratory (#PopHealthLab), University of Fribourg, Switzerland

References
1) Mahase E. Monkeypox: What do we know about the outbreaks in Europe and North America? BMJ 2022; 377:o1274
2) World Health Organization. WHO Health topics: Monkeypox. Available from https://www.who.int/health-topics/monkeypox#tab=tab_1, accessed on May 27, 2022
3) Mathieu E, Dattani S, Ritchie H. Roser M. Ourworldindata: Monkeypox. Available from https://ourworldindata.org/monkeypox, accessed on May 27, 2022
4) Chiolero A, Buckeridge D. Glossary for public health surveillance in the age of data science. JECH 2020; 74(7): 612–616.

Competing interests: No competing interests

27 May 2022
Stefano Tancredi
PhD student
Arnaud Chiolero
Population Health Laboratory (#PopHealthLab), University of Fribourg, Switzerland
Fribourg 1700