Q&A: Waning smallpox immunity not the only cause of mpox surge in DRC
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Key takeaways:
- The end of smallpox vaccination in the Democratic Republic of Congo is responsible for some of the mpox surge there.
- Spillover events and increased transmissibility are contributing to its spread.
Waning smallpox immunity in the Democratic Republic of the Congo has contributed to a significant increase in mpox cases over the last 15 years, but a study suggests there are also other causes for the virus’ rise in prominence.
“It’s quite strange that hotspots are emerging in parts of the country that are mainly savannahs and grasslands because, in the DRC, mpox was previously a disease reported in highly forested regions,” Eugene Bangwen, MSc, MPH, scientific fellow in outbreak research at the Institute of Tropical Medicine in Antwerp, said in a press release related to the study.
WHO in August declared mpox spread in Africa a global public health emergency, based on the spread of clade I mpox in eastern DRC and other countries in Africa. The emergency declaration came 15 months after the agency ended the public health emergency declared over the 2022 global outbreak of clade IIb mpox.
According to the study presented at the annual meeting of the American Society of Tropical Medicine and Hygiene, Bangwen and colleagues conducted a retrospective analysis of data on more than 60,000 cases mpox cases and 1,800 deaths, a case fatality of 2.9%, reported by the DRC between 2010 and 2023.
The researchers reported that suspected cases in the DRC increased to 11.5 per 100,000 people in 2023 from 2.9 per 100,000 people in 2010, with the highest incidence (46.46 per 100,000) and fatality (5.9%) rates seen in children aged 5 years and younger.
During the 14-year study period, the number of annual cases reported by the DRC ranged from 2,302 cases in 2010 to a high of 14,636 cases in 2023, with the lowest reported number of cases occurring in 2011 (2,211).
According to the study, some portion of the increase in mpox cases in the DRC and other parts of central and eastern Africa are linked to waning smallpox vaccination, which can protect against mpox.
Vaccination efforts ended in 1980 after the eradication of smallpox, and the number of mpox cases in the DRC started to rise — including the four-fold rise seen in the last 14 years — which is why so many younger people and children have been infected, the researchers wrote.
The study notes, however, that new spillover events and increased transmissibility are also likely to be responsible for the rise in mpox cases.
Healio asked Bangwen about the study’s results and what they mean for the future of mpox.
Healio: The cessation of smallpox vaccination has been linked to mpox issues in the DRC. Do you agree or disagree?
Bangwen: I wouldn’t say I disagree. On the contrary, our study tries to investigate whether the cessation of smallpox vaccination might have an effect on the emergence of mpox as has been long hypothesized in the literature. We did this by investigating age trends in laboratory confirmed mpox patients in the DRC. What we found was that the median age group remains relatively stable over time, but however there has been a steady increase in the upper limit of the age of the oldest affected individuals over the years. This might mean that the population of immunologically naïve individuals who are susceptible to mpox has increased. That is therefore suggestive of the fact that cessation of smallpox vaccination which conferred cross-protective immunity to mpox might rather be linked to emergence of mpox in the DRC.
Healio: The abstract mentions that spillover events contribute significantly to the DRC’s ongoing mpox issues. What type of events would these be?
Bangwen: Spillover events refer to instances where the monkeypox virus is transmitted from an animal source to a human. Usually this is followed by a series of human-to-human transmission cycles. In the DRC, this has been reported to be the primary means by which outbreaks of mpox begin. Very often, this is due to contact with wild animals especially in highly forested areas. These events are a significant concern in regions like the DRC due to close human contact with wildlife, as well as ecological and socio-economic factors that increase the likelihood of such interactions.
Healio: How does mpox in the DRC relate to or affect the potential for another global mpox outbreak?
Bangwen: Before the global outbreak of clade IIb, the DRC had reported the highest number of mpox suspected case notifications, followed by Nigeria. Clade II, however, had been exported to previously non-endemic countries in the west but was quickly controlled and did not lead to large outbreaks. However, when clade IIb was found in non-endemic countries in 2022, no one expected that it would case a worldwide outbreak of such magnitude.
The world is now seeing a gradual trend where clade I, which had never been exported to Western countries, has for the first time been detected, not only cross-border from the DRC, which were previously non-endemic, but also in a few Western countries. I am not saying that there would be another global outbreak, but it is indeed possible that if not properly managed, history may repeat itself.
Healio: What is the main takeaway from the study?
Bangwen: The main takeaway from the study is that there has been an exponential rise in the incidence of mpox with previously non-endemic health zones reporting cases and that this may be related to the waning immunity of smallpox vaccination compounded by other ecological factors.
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Bangwen E, et al. Session Abstract LB-9328. Presented at: American Society of Tropical Medicine & Hygiene; Nov. 13-17, 2024; New Orleans.
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