#18,290
Mpox (formerly Monkeypox) has simmered for decades in central Africa, but over the past dozen years has shown signs of increasing transmission. In 2013 the DRC reported a 600% increase in cases over both 2011, and 2012.
A 2016 study (see EID Journal:Extended H-2-H Transmission during a Monkeypox Outbreak) suggested that the virus's epidemiological characteristics may be changing (possibly due to the waning smallpox vaccine derived immunity in the community).
Like all viruses, Monkeypox continues to evolve and diversify (see 2014 EID report Genomic Variability of Monkeypox Virus among Humans, Democratic Republic of the Congo), where the authors cautioned that small genetic changes could favor adaptation to a human host.
As might be expected, the number of Mpox variants circulating in central Africa continues to grow, with the recent addition of another subclade; Ib (see Preprint: Sustained Human Outbreak of a New MPXV Clade I Lineage in Eastern Democratic Republic of the Congo).
While there are conflicting reports on the severity and case fatality rate of this newly identified clade, by all accounts it is more severe than the clade IIb mpox, which began its world tour in 2022.
This new Ib subclade also appears to be transmitting more readily, across more groups, and by more means than clade IIb. Parsing out these differences, however - particularly in regions of the world where testing and surveillance are often lacking - can be a daunting task.
All of which brings us to a new EID Journal report which finds evidence that this new clade has increased its reproductive number R0 (r-nought) to above the epidemic threshold (> 1.0).
Research
Epidemiologic Quantities for Monkeypox Virus Clade I from Historical Data with Implications for Current Outbreaks, Democratic Republic of the Congo
Valentina Marziano, Giorgio Guzzetta, Ira Longini1, and Stefano Merler1
Abstract
We used published data from outbreak investigations of monkeypox virus clade I in the Democratic Republic of the Congo to estimate the distributions of critical epidemiological parameters. We estimated a mean incubation period of 9.9 days (95% credible interval [CrI] 8.5–11.5 days) and a mean generation time of 17.2 days (95% CrI 14.1–20.9 days) or 11.3 days (95% CrI 9.4–14.0 days), depending on the considered dataset. Presymptomatic transmission was limited.
Those estimates suggest generally slower transmission dynamics in clade I than in clade IIb. The time-varying reproduction number for clade I in the Democratic Republic of the Congo was estimated to be below the epidemic threshold in the first half of 2024.
However, in the South Kivu Province, where the newly identified subclade Ib has been associated with sustained human-to-human transmission, we estimated an effective reproduction number above the epidemic threshold (95% CrI 0.96–1.27).
(SNIP)
The epidemiology of MPXV clade I seems to be evolving; until recently, it was mainly of zoonotic origin, and only sporadic human-to-human transmission occurred within households. Sexual transmission of MPXV clade I was observed during an outbreak investigation in Kwango Province, DRC, in March 2023 (3).
A new subclade has been recently identified in Kamituga in the South Kivu Province, where extensive sexual transmission has been documented (4–6). Sustained human-to-human transmission, new routes of infections, and genetic evolution raise concerns about the potential risk for international spread, making estimating critical epidemiologic parameters specific to MPXV clade I urgent.In this study, we used previously published outbreak investigation data to provide novel estimates of probability distribution functions for epidemiologic parameters that are critical for monitoring and modeling the spread of MPXV clade I (K. Charniga et al., unpub. data, https://arxiv.org/abs/2405.08841 External Link).
(SNIP)
Discussion
The estimated mean incubation period of MPXV clade I (9.9 days, 95% CrI 8.5–11.5 days) was longer than most estimates reported for MPXV clade IIb in a review by the World Health Organization (17) (although some of the studies reported in the review might use a different definition of incubation period [e.g., from exposure to first symptoms rather than from exposure to rash as in our data]). Available data, although limited in sample size, point to a variability in serial intervals/generation times depending on the observation settings, with cases from household outbreaks having mean values ranging from 14 to 21 days (mean 17 days) and cases from an outbreak associated to hospital cases ranging from 9 to 14 days (mean 11 days).
The transmission dynamics of MPXV clade I seem slower than clade IIb, for which mean estimates range from 5.6 to 9.4 days for serial intervals (17) and ≈12.5 days for generation times (12). We suggest a limited occurrence of presymptomatic transmission, on the basis of the estimated distribution of the infectious period and available data on serial intervals. This finding is again in contrast with MPXV clade IIb, for which substantial presymptomatic viral shedding and transmission has been demonstrated (18,19).
The estimated Rt for MPXV clade I in the DRC in 2024 has been hovering around values that are compatible with historical estimates of 0.75–0.86 (20–22), below the epidemic threshold of 1.
However, the Rt estimated for the Kamituga Health Zone, South Kivu province, where a recently identified subclade has emerged (5), was above threshold; the mean value of Reff was from 1.08 to 1.18 (95% CrI 0.96–1.27).
This substantially higher value, compared with historical estimates, might be caused by possible viral adaptation to human transmission, as well as by higher local contact rates because of sexual transmission given the concentration of sex workers associated with the mining industry in the region (L.M. Masirika et al., unpub. data).
For years we've seen researchers warn that if left unchecked, the mpox epidemic in central Africa could eventually evolve into a much more formidable - and potentially global - threat. Those warnings (see PLoS NTD: The Changing Epidemiology of Human Monkeypox—A potential threat?), were largely ignored.
In the spring of 2022, a milder clade IIb Mpox virus sparked a global emergency, and while that emergency was canceled in 2023, that virus continues to spread internationally.
While the threat from clade Ib remains poorly defined, it does appear to be far more serious than clade IIb. How much more, remains to be seen.
But given the on-going zoonotic spillovers of Mpox, and the continued evolutionary forces at play, there is no guarantee that clade IIb will be the last, or even the worst, version the world will have to deal with.
Stay tuned.
