Global immuno-epidemiology and the persistence of mpox clade IIb in MSM

Viruses do not respect borders, and neither do the people who carry them. When a disease spreads through a specific group of people, its ability to keep circulating depends on a delicate balance between how many people are still vulnerable to infection and how many have built up defenses. This balance is shaped by two main forces: the arrival of new cases from outside and the gradual fading of immunity within the population. If a virus runs out of new hosts to infect, it usually dies out. But if the population is large enough and connected enough to the wider world, the virus can find a way to hang on, waiting for the right moment to return. Understanding how this happens is crucial for public health, because it tells us when a disease is truly gone and when it is merely hiding, ready to resurface.

In 2022, a strain of the mpox virus, known as clade IIb, began spreading rapidly among men who have sex with men across the globe, moving primarily through sexual contact. By the summer of that year, the number of new cases in Europe and North America began to drop sharply. This decline happened because people changed their behavior to avoid infection and because many received vaccines. By 2023, reported cases in these regions had nearly vanished, leaving behind only a few scattered instances of transmission. This sudden quiet raised a puzzling question for scientists: if the virus seemed to have disappeared from these major hubs, how did it manage to survive and eventually settle into a long-term, low-level presence? The virus only stays infectious in a person for a few weeks, so keeping a chain of transmission going requires a constant flow of new infections. If that flow stops, the chain should break.

To solve this mystery, researchers built a detailed computer simulation of how the virus moves through the sexual contact networks of men who have sex with men in Berlin. This model acted like a digital laboratory, tracking how the virus spreads from person to person while accounting for how long people shed the virus, how vaccines work, how immunity fades over time, and how new cases arrive from other places. When the team ran their simulation for Berlin alone, the results showed that the virus should have died out. In fact, the model estimated that there was more than an 80 percent chance the virus would have gone extinct in Berlin during 2023, simply because the people who were still vulnerable were too few and too scattered to keep the chain going.

However, the story changed when the researchers expanded their view. They connected the Berlin simulation to a larger network representing men who have sex with men across all of Europe, and then to a global network. As the scope widened, the chance of the virus disappearing dropped dramatically. In the European network, the likelihood of extinction fell below 60 percent. When the entire world was included in the model, the probability of the virus dying out in Berlin dropped to nearly zero. The key to this survival was a phenomenon called epidemic asynchrony. While the virus was fading in Europe and the Americas, it was still actively spreading in parts of Asia. This difference in timing meant that as immunity in Europe began to fade and the population became vulnerable again, the virus was still circulating elsewhere. It then traveled back to Europe, re-establishing the chains of transmission that had almost broken.

This pattern of persistence is not just a theoretical possibility; it matches what genetic testing of the virus has revealed. The researchers found that the active clusters of mpox in Germany are linked to two specific sources: either the virus was brought back from the Americas, or it emerged from new strains that originated in Asia. The virus did not vanish and then magically reappear; it was kept alive by a global cycle where it burned out in one region while thriving in another, only to be reintroduced later. The study suggests that the persistence of mpox is a direct result of how these global networks are connected. It highlights that controlling such a disease requires more than just local efforts; it demands coordinated international surveillance, because the virus can hide in one part of the world and return to another when the time is right.