The changing epidemiology of human monkeypox—A potential threat? A systematic review

If you want to understand why a virus most people had never heard of suddenly mattered everywhere, start with a success story. When the world declared smallpox eradicated in 1980, routine vaccination ended. That shot didn't just protect against smallpox; it also offered strong cross-protection against its cousins. Historical data put that protection around 85 percent for monkeypox. As vaccinated generations aged and new cohorts grew up unvaccinated, our collective shield thinned. That's the backdrop for the resurgence you've been seeing. To trace how that story unfolded, Bunge and colleagues built a panoramic review of human monkeypox since the first recognized case in 1970 in what is now the Democratic Republic of the Congo, or DRC. They registered the plan in the International Prospective Register of Systematic Reviews, known as PROSPERO, followed Preferred Reporting Items for Systematic Reviews and Meta-Analyses, known as PRISMA, guidance, and pulled together 48 peer-reviewed papers and 18 reports from what we call grey literature. The sweep is wide: ten African countries with reported activity and four countries outside Africa with documented outbreaks or importations. It's not just a tally. The review pulls on threads of age, geography, fatality, and how people actually get infected. A quick word on how they navigated messy data. There were no language limits, and they went beyond PubMed and Embase to African journals and outbreak bulletins from the World Health Organization, or WHO, and the Centers for Disease Control and Prevention, known as the CDC. Case definitions weren't uniform across decades, so they harmonized categories like suspected, probable, and confirmed where they could. Clade—the genetic subgroup that matters for severity—wasn't consistently reported, so they assigned clades by geography following WHO guidance, with Cameroon set aside because both clades circulate there. That choice adds uncertainty, and they say so. But it lets them compare broad patterns across time and place in a way a single outbreak investigation cannot. The first pattern is the simplest to grasp: more cases over time, centered in Central Africa. In the 1970s, 48 confirmed and probable human cases were recorded across six countries, most of them in the DRC. By the 1980s, the DRC alone reported 343. The 1990s continued the climb with 511 in the DRC. Enter the 2000s and 2010s, and you see hundreds of confirmed cases alongside surging suspected counts—nearly nineteen thousand suspected cases were logged in the 2010s—reflecting both real transmission and the changing intensity of surveillance. The second pattern is geographic reach. In 2003, the first outbreak outside Africa hit the United States: 47 confirmed or probable cases tied to pet prairie dogs infected by rodents imported from Ghana. Travel then became the vector for tiny sparks elsewhere—Israel in 2018, the United Kingdom, or UK, in 2018 and 2019, and Singapore in 2019. Inside Africa, Nigeria lit up after a long quiet period. Beginning in 2017, Nigeria's public health agency reported 181 confirmed or probable cases in the first year, and linked travel seeded a handful of cases in the UK and Israel. One UK case even spread in a hospital to a healthcare worker, a reminder that close contact can trump borders. Who gets sick also changed. Monkeypox used to be mainly a childhood disease. In the 1970s and 1980s, the median age at infection sat around four to five years. It moved to about ten years in the 2000s, then to 21 years in the 2010s. That shift points straight back to smallpox vaccination. Older adults who had been vaccinated decades earlier kept some immunity; younger adults and children did not. Across many outbreaks, men and women were affected in roughly similar proportions, though outside Africa, adult men were overrepresented in the travel-linked cases. You can see the population-level version of that story in the immunity landscape. As Bunge and colleagues summarize from historical vaccination coverage, smallpox-derived immunity in the general population fell from about 65.6 percent in 1970 to roughly 2.6 percent by 2016 and 2.2 percent by 2018. On the ground, that translated into case rosters dominated by people without prior smallpox vaccination—typically 80 to 96 percent of cases in the outbreaks where vaccination status was reported. That's not a subtle effect. It's a shift in the basic conditions under which the virus spreads. Zoom in, and you can watch that protection play out in numbers. In the Central African Republic, attack rates were lower among people with a smallpox vaccination scar—just under one per 1,000—than among those without it, where the rate was about 3.6 per 1,000. In the 2003 outbreak in the U.S., where most patients were too young to have been vaccinated, about 21 percent of cases had received a smallpox shot in the past. Even there, the balance tilted heavily toward the unvaccinated. The immunological message is just consistent: vaccinia-based immunity matters. Severity, however, is not uniform, because not all monkeypox viruses are alike. Two main genetic clades circulate, and they've been associated with different clinical outcomes. Pooling across countries, the review estimates an overall case fatality rate of 8.7 percent. Split by clade, the Central African clade sits higher at 10.6 percent, while the West African clade is lower at 3.6 percent. Those are averages across patchy data, but the contrast is robust. Who dies has shifted with time as well. In the 1970s through the 1990s, every recorded death in Africa occurred in children under ten. In the 2000s and 2010s, only about 37.5 percent of deaths were in that youngest group, with more fatalities among adolescents and adults. Outside Africa, deaths were rare in the pre-2022 literature, with a notable exception of that nosocomial case in the UK. How do people catch it? The answer has always been "it depends," but you can see a tilt from animal to human sources as immunity waned and the virus found more susceptible hosts. In the DRC during the 1980s, animal exposure was suspected in about 72.5 percent of cases and human-to-human transmission in 27.5 percent. By the 1990s, roughly 78 percent of cases were secondary—meaning patients had contact with another infected person. In Nigeria's 2017 to 2018 outbreak, many patients couldn't pin down an exact source, but among those who could, most had an epidemiological link to someone with similar lesions and only a small fraction reported animal contact. The behaviors that show up repeatedly are intimate and practical: sleeping in the same room or bed, sharing dishes, caring for a sick relative. If you want a sense of how efficiently it can move through households, look at secondary attack rates—the chance a close contact gets infected from a primary case. Across 16 peer-reviewed studies, nine found no secondary spread among traced contacts. Six reported modest rates, generally between about 0.3 percent and 10 percent. And one exceptional cluster, spanning 16 households, recorded a median secondary attack rate of 50 percent. That outlier is a reminder that context—household crowding, care practices, concurrent conditions—can bend the average in dramatic ways. Incidence data, where surveillance was strong enough to produce them, underline the upward pressure. In national DRC surveillance between 2001 and 2013, reported incidence rose from about 0.64 per 100,000 people to 2.82 per 100,000. In the Central African Republic, an outbreak in 2016 reached an attack rate around 50 per 10,000 in the affected area, orders of magnitude above background. Those numbers don't just mean more virus; they also reflect when and where people were looking. Surveillance wasn't constant across decades or districts, so any single rate should be read with caution, but the direction of travel is clear. Could it sustain spread without new animal introductions? Modeling suggests the answer can be yes. Nguyen and colleagues, working from outbreak data, estimated a basic reproduction number—R naught—above one in some settings. R naught above one means each case, on average, creates more than one new case in a fully susceptible population. In other words, epidemic potential is present, not just a theoretical curiosity. All of this sits on a wobbly foundation of data, and the authors are candid about that. After the World Health Organization's formal surveillance ended in the DRC in the mid-1980s, standardized national counts became sparse; later systems like Integrated Disease Surveillance and Response didn't capture the same details everywhere. Case definitions varied by country and year. Clade assignments often had to be inferred from geography rather than genetic testing. And grey literature—outbreak bulletins and rapid reports—filled in gaps but wasn't designed for pooled analysis. The likely effect of those limitations is underestimation: fewer cases captured, less clarity about who infected whom, and wider uncertainty around fatality differences by clade. Even with those caveats, the arc is coherent. As Bunge, Hoet, Chen, and colleagues show, ending smallpox vaccination left a growing pool of people without orthopoxvirus immunity. Over the following decades, monkeypox cases increased, the median patient got older, and the virus reached further afield through travel and trade. Severity differed by clade, but in both, human-to-human transmission became more prominent as the immunological landscape changed. And the occasional international sparks—from prairie dogs in the Midwest to nosocomial spread in a British hospital—made it clear this wasn't just a local forest virus anymore. So where does that leave us? The authors are pragmatic. They don't call for moonshots; they call for basics done well. Better surveillance, so suspected cases aren't just tallied but confirmed and mapped. Consistent case definitions and contact-tracing, so secondary attack rates mean the same thing in Sankuru as they do in Abuja. Genetic typing, so clade assignments aren't proxies. And international coordination, because the chain that runs from a rodent in Ghana to a pet store in Illinois is built from policy decisions as much as biology. Pull the lens back one more time, and the lesson is bigger than one virus. Eradication is a triumph, but it also rewrites the ecosystem of immunity. Forty years later, we're living in that new ecosystem. The numbers in this review—an overall case fatality of 8.7 percent, a median age that rose from kindergarten to early adulthood, an R naught that can tip above one—aren't just statistics. They're signals. And they're telling us that when we lower the defenses that used to be there by accident of another vaccine, we need to build new ones on purpose. That starts with seeing clearly, which is what this synthesis makes possible.