Unfortunately, historical assumptions have proven wrong in 2022. While 94 percent of patients in a Belgian cohort showed characteristic skin lesions, detailed analysis of UK data linking cases and contacts showed that most cases were transmitted before they became symptomatic. That’s significantly more than previously thought.
By the end of 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 vaccination rollout had fully got underway.
Subsequent analysis of the underlying transmission factors showed that the reduction in this outbreak was likely due to the build-up of immunity after infection in the groups involved in the outbreak, combined with behavioral changes that reduced further transmission:
Our model-based analysis shows that the most likely reason the Mpox epidemic turned ugly in the UK was a combination of high population exposure among the small number of people in the most sexually active groups and behavioural change that resulted in a relatively lower risk of onward transmission from infected people.
A new PHEIC
Which brings us to 2024. While outbreaks have been spreading in recent months, the threat has been looming for some time. During the 2022 clade 2 outbreak, 200 deaths were reported in countries that historically had not reported MPOX outbreaks. Yet during the 2023 clade 1 outbreak in the DRC, over 1,200 deaths were reported, and for the first time in clade I, sexual contact was estimated to be a significant route of transmission. In 2024, researchers also identified a new “clade 1b” subvariant that was responsible for much of the ongoing transmission in the DRC. By the end of July 2024, an additional 2,600 confirmed cases and 450 deaths had been reported, with 68 percent of cases and 85 percent of deaths occurring in people under the age of 15.
It is unlikely that this mortality rate is due to the age structure of the population alone. Earlier this year, an analysis of systematic reviews of historical outbreaks of clade 1 estimated a much higher mortality rate among the youngest age groups.
There is also the question of how widespread transmission might be. For example, on 25 July 2024, Burundi reported a new clade 1b mpox outbreak. As of 14 August, 23 of 49 districts were affected, with a total of 103 confirmed cases (and 464 suspected cases).
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 mortality risk for this clade is relatively low, a pre-print published earlier this week estimated that the true size of the outbreak could be as many as 290-560 cases among MSM in the country.