A study in Cell in February 2024 explores mpox transmission and population-level changes associated with controlling the spread after the public health emergency of international concern (PHEIC) that was declared in July 2022. The authors find “community transmission prior to detection”, changes to case reporting during the epidemic, and a “large degree of transmission”. They also find that “viral introductions played a limited role in prolonging spread after initial dissemination”, and that mpox transmission in North America started to decline “before more than 10% of high-risk individuals in the USA had vaccine-induced immunity”.
“Our findings highlight the importance of broader routine specimen screening surveillance for emerging infectious diseases and of joint integration of genomic and epidemiological information for early outbreak control.”
Mpox
Mpox, previously known as monkeypox, is a viral zoonotic disease that is caused by the mpox virus (MPXV). It is endemic to West and Central Africa; the paper states that most cases of mpox outside these regions were identified in individuals with a recent travel history to Nigeria or an exposure to live animals from endemic areas.
On 7th May 2022 an individual with a travel history to Nigeria was diagnosed with mpox in the UK. After this case, the number of cases without a travel history to endemic countries “began to increase rapidly in various reasons” in a way that was “consistent with epidemic human-to-human spread”. By July 2023 the CDC had recorded 88,549 global cases since January 2022. The epidemic was characterised by human-to-human transmission “outside of endemic areas”, mostly in men who have sex with men (MSM).
The cases were defined by a “less severe illness presentation” in comparison with historical short human-to-human transmission chains and the long incubation period of 5-21 days suggests that “mpox may have spread undetected prior to initial case discovery”. Furthermore, presymptomatic transmission of mpox has been document, which implies that the epidemic was “at least partially fuelled by transmission occurring prior to symptom onset”.
Interventions and investigations
When WHO declared mpox a PHEIC countries began investigating disease spread, the use of vaccines, and potential guidelines on international travel. However, there were criticisms of the delay in starting “effective vaccination campaigns in high-risk areas”. The authors believe that genomic epidemiology is “uniquely poised” to explore “global and regional transmission dynamics” through a “joint integration of viral genomic information and epidemiological metadata”. This “augments traditional public health surveillance”.
Therefore, the study uses advances in phylogeographic and phylodynamic methods to estimate changes in case detection rate, the effect of “underdetection” on transmission, and the role of “introductions in promoting local community spread in various global regions”. Furthermore, they examine the consequences of vaccination campaigns on epidemic “growth and decay” in North America and estimate the degree of transmission heterogeneity in the declining phase of the epidemic.
The study’s findings
The study presents a “global and regional view” of mpox detection, expansion, and containment. Among their findings, the authors highlight “limited impact of vaccination campaigns during the early phases of the North American epidemic”. They compared changes in local transmission to the cumulative percentage of high-risk individuals in the US with vaccine-derived immunity. Despite the effectiveness of even half a vaccination dose in providing “robust immunity” against mpox, there was “concern over the delayed start of vaccination campaigns in the US”.
The conclusions that they draw are “concordant” with CDC conclusion, finding that Rt fell below one in August 2022 when “only about 1.3% of the high-risk population” in the US had any vaccine-induced immunity. Mpox modelling in Washington DC indicates that “behavioural modifications” within the MSM community were the “main contributing factor to slowing initial mpox spread”. However, vaccination campaigns were “ultimately needed to definitively curb the local epidemic and prevent future outbreaks”.
In the UK, a study that focused on MSM found that “vaccination could not explain the drop in mpox incidence in the region”. Instead, it attributes the declining incidence to “changes in behaviour”.
“These findings highlight the significant effect of behavioural change among MSM in curbing the epidemic as well as emphasise the need for prompt public health response in order to maximise the population-level effectiveness of vaccination campaigns.”
What implications does this study have for future public health efforts, and how might it influence public health messaging as well as public health interventions? For more on disease management with vaccination and to participate in discussions about lessons from and for mpox, get your tickets to the Congress in Washington in April. Don’t forget to subscribe to our newsletters for more insights!
