Unfortunately, historical assumptions did not always hold up in 2022. Although 94 per cent of patients in a Belgian cohort displayed characteristic skin lesions, a detailed analysis of UK data that linked cases and contacts suggested the majority of cases transmitted before they became symptomatic, considerably more than previously thought.
By late September 2022, around 40,000 vaccine doses had been distributed to at-risk groups in the UK. However, the outbreak peaked in July 2022, before the vaccine rollout had fully got going.
A later analysis of underlying transmission drivers suggested that this outbreak decline was likely to be down to accumulation of post-infection immunity in the groups connected to the outbreak, combined with behaviour change reducing onwards transmission:
Our model-based analysis suggests that the most likely reason the Mpox epidemic in the United Kingdom turned over was a combination of high population exposure among the small number of people in the most sexually active groups and behaviour change resulting in relatively lower risk of forward transmission from infected people.
A new PHEIC
Which brings us to 2024. Although outbreaks have spread in recent months, the threat has been looming for a while. During the 2022 clade 2 outbreak, there were 200 deaths reported in countries that had not historically reported mpox outbreaks. Yet during the 2023 clade 1 outbreak in the DRC, there were more than 1,200 deaths reported, and for the first time for clade I, sexual contact was estimated to be a key driver of transmission. In 2024, researchers also identified a new ‘clade 1b’ subvariant that was behind much of the sustained transmission in DRC. Up to the end of July 2024, an additional 2600 confirmed cases and 450 deaths were reported, with 68 per cent of the cases and 85 per cent of the deaths among under 15s.
It’s unlikely this fatality pattern is just down to population age structure; earlier this year, an analysis of systematic reviews of historical clade 1 outbreaks estimated a much higher fatality risk among the youngest age groups.
There’s also the question of how widespread transmission might be. For example, on 25th July 2024, Burundi reported a new clade 1b mpox outbreak. By 14th August, 23 out of 49 districts were affected, with 103 confirmed cases in total (and 464 suspected).
Then there’s the issue of ongoing clade Ia and clade 2b transmission. For example, between May and July 2024, South Africa reported 22 cases and three deaths of clade 2b. Because the fatality risk for this clade is comparatively low, a pre-print published earlier this week estimated that the true outbreak size could be as large as 290–560 cases among MSM in the country.
