Description of the first global outbreak of mpox: an analysis of global surveillance data

Abstract

Background: In May 2022, several countries with no history of sustained community transmission of mpox (formerly known as monkeypox) notified WHO of new mpox cases. These cases were soon followed by a large-scale outbreak, which unfolded across the world, driven by local, in-country transmission within previously unaffected countries. On July 23, 2022, WHO declared the outbreak a Public Health Emergency of International Concern. Here, we aim to describe the main epidemiological features of this outbreak, the largest reported to date.

Methods: In this analysis of global surveillance data we analysed data for all confirmed mpox cases reported by WHO Member States through the global surveillance system from Jan 1, 2022, to Jan 29, 2023. Data included daily aggregated numbers of mpox cases by country and a case reporting form (CRF) containing information on demographics, clinical presentation, epidemiological exposure factors, and laboratory testing. We used the data to (1) describe the key epidemiological and clinical features of cases; (2) analyse risk factors for hospitalisation (by multivariable mixed-effects binary logistic regression); and (3) retrospectively analyse transmission trends. Sequencing data from GISAID and GenBank were used to analyse monkeypox virus (MPXV) genetic diversity.

Findings: Data from 82 807 cases with submitted CRFs were included in the analysis. Cases were primarily due to clade IIb MPXV (mainly lineage B.1, followed by lineage A.2). The outbreak was driven by transmission among males (73 560 [96·4%] of 76 293 cases) who self-identify as men who have sex with men (25 938 [86·9%] of 29 854 cases). The most common reported route of transmission was sexual contact (14 941 [68·7%] of 21 749). 3927 (7·3%) of 54 117 cases were hospitalised, with increased odds for those aged younger than 5 years (adjusted odds ratio 2·12 [95% CI 1·32-3·40], p=0·0020), aged 65 years and older (1·54 [1·05-2·25], p=0·026), female cases (1·61 [1·35-1·91], p<0·0001), and for cases who are immunosuppressed either due to being HIV positive and immunosuppressed (2·00 [1·68-2·37], p<0·0001), or other immunocompromising conditions (3·47 [1·84-6·54], p=0·0001).

Interpretation: Continued global surveillance allowed WHO to monitor the epidemic, identify risk factors, and inform the public health response. The outbreak can be attributed to clade IIb MPXV spread by newly described modes of transmission.

Funding: WHO Contingency Fund for Emergencies.

Translations: For the French and Spanish translations of the abstract see Supplementary Materials section.

Figure 1

Mpox confirmed cases reported globally and their main characteristics, from Jan 1, 2022, to Jan 29, 2023 (A) Epidemic curves in all WHO regions. Note different y-axis scales for each region. The European region and the region of the Americas reported by far the highest number of cases with declining case counts at the end of this reporting period. The African region shows an unclear trend due to inconsistent reporting frequencies and more diverse patterns of transmission; other regions had largely sporadic cases and clusters. (B) Age and sex distribution of all cases. Cases were majority male and were most common among those aged between 18 and 50 years. (C) Frequency of symptoms among population groups stratified by sex and age. Of note, frequency of genital rash varied widely between population groups. Note that the skin or mucosal lesions exclude oral and genital lesions.

Figure 2

Phylogenetic trees of available monkeypox virus genomic sequences in public databases to Sept 20, 2022 (A) Genomic sequences from Jan 1, 2022, until Sept 20, 2022, highlighted with coloured tips according to WHO region. Outside of central Africa, all sequences are associated with clade IIb, with the majority falling under lineage B.1. (B) Genomic sequences sampled before 2022 shown with coloured tips by country, highlighting differences between cases in clade I and II. Countries described under other countries are located outside of the African continent, which had sporadic mpox cases before 2022.

Figure 3

Trends in confirmed cases over time for the two WHO regions where modelling inclusion criteria were met, with each line representing a single country Weeks in which countries reported less than ten confirmed cases have been excluded due to uncertainty surrounding estimates based on low case numbers. (A) Daily incidence rate for countries shown as number of confirmed cases per 100 000 population, by modelled date of infection, split by region to show regional differences in trends. (B) Weekly incidence rates for countries shown as the number of cases per 100 000 population, shown by week of report. Incidence was estimated using population estimates from the UN Population Division. (C) Confirmed cases over time, by modelled date of infection, shown as a percentage of the peak number of infections. (D) Median estimated R_eff over time for countries which met the inclusion criteria. R_eff=effective reproduction number.