The findings come from a collaborative study involving Nigeria’s Institute of Human Virology, the University of Cambridge and other partners, published in Nature Communications. While the researchers emphasise that the data do not point to a large, hidden epidemic, they warn that low-level, undetected transmission may be more common than previously understood.
The study arrives at a critical moment for Nigeria’s public health strategy. In 2024, Nigeria became the first African country to begin rolling out mpox vaccines, with vaccination plans largely informed by reported symptomatic cases. The new evidence raises questions about whether symptom-based surveillance alone provides a complete picture of how the virus is spreading.
Mpox is a zoonotic disease caused by the MPXV virus and can spread through animal-to-human transmission or close person-to-person contact. Classic symptoms include fever, swollen lymph nodes and a painful rash. For decades, mpox transmission was partially suppressed by widespread immunity from routine smallpox vaccination, which also provided protection against mpox. That global vaccination programme ended in 1980 after smallpox was eradicated.
Since then, a growing share of the population—particularly younger generations—has lacked this cross-protective immunity. In Nigeria and other parts of West and Central Africa, this immunity gap has coincided with renewed mpox transmission, including multiple outbreaks recorded between 2022 and 2024.
To assess whether the virus might be spreading beyond what clinical data suggest, researchers analysed archived blood samples from 176 healthy Nigerian adults. The participants had originally been enrolled in SARS-CoV-2 vaccine studies and included healthcare workers sampled in 2021 and community volunteers sampled in 2023.
None of the participants had received mpox or smallpox vaccination in adulthood, and none were known to have had mpox. Analysis showed that 14 per cent of participants carried antibodies linked to historic smallpox vaccination. These antibodies were concentrated among people born before 1980, highlighting what researchers describe as a generational “immunity gap”.
More striking, however, was evidence of recent, previously undetected exposure. When follow-up blood samples were collected—typically around nine months later—about 3 per cent of participants showed clear signs of a new immune response consistent with recent mpox infection. None of these individuals had been diagnosed with mpox or reported symptoms associated with the disease.
“What we are seeing is evidence that mpox exposure doesn’t always look like the textbook description,” said Adam Abdullahi, the study’s lead author, speaking to SciDevNet. “In some people, particularly in settings with partial population immunity, the virus may circulate quietly, leaving immune footprints that routine clinical surveillance will miss.”
To place these findings in a broader context, the research team also analysed more than 100 mpox genomes collected across Nigeria. The genetic data suggest that while the virus continues to circulate, its spread is constrained by residual immunity in older populations.
Ravindra Gupta, a senior author of the study from the Cambridge Institute of Therapeutic Immunology and Infectious Disease, said the pattern points to sustained low-level transmission rather than explosive spread. “Mpox is not spreading unchecked in Nigeria and across the region, but neither is it absent,” he said. “Instead, it appears to circulate at low levels, shaped by the lingering effects of smallpox vaccination in older generations.”
The researchers also found no meaningful difference in immune responses between healthcare workers and the general population, suggesting that exposure is not confined to clinical or hospital settings.
The findings highlight a potential blind spot in Nigeria’s current mpox surveillance system, which—like many across Africa—relies primarily on identifying people who seek care for visible symptoms. While this approach is effective for detecting obvious outbreaks, it may miss low-level or asymptomatic transmission occurring in the community.
Experts say addressing this gap would require a shift in surveillance strategy. Alash’le Abimiku, executive director of the International Research Center of Excellence at the Nigerian Institute of Human Virology, said antibody-based surveillance could play a critical role.
“Instead of relying solely on reported cases or symptoms, monitoring populations by testing blood samples for antibodies can reveal exposure to the virus and help us understand how it is spreading,” she said. “This is important for guiding targeted vaccination in our settings.”
Abdullahi echoed that view, noting that people with silent infections are unlikely to visit health facilities and therefore remain invisible to traditional monitoring systems.
“If we only look for obvious disease, we will miss part of the picture,” he said. “Monitoring blood samples allows us to see transmission that does not result in clinic visits, especially in regions where health systems are already stretched and require strengthening.”
Together, the researchers argue, the findings support a more nuanced approach to mpox control in Nigeria—one that combines clinical reporting with immunological surveillance to better capture how the virus is circulating in the population.