Study raises alarm over silent mpox transmission in Nigeria

Scientists have warned that the mpox virus may be circulating silently in parts of Nigeria, often without symptoms typically associated with the disease.

A study published in Nature Communications found that exposure to the mpox virus can occur without recognised illness and that residual immunity from historic smallpox vaccination continues to influence the spread of the virus in human populations.

Researchers from the University of Cambridge, in collaboration with Nigerian partners, said that this under-recognised transmission among healthy Nigerian adults has significant implications for public health surveillance.

Current mpox monitoring largely relies on the detection of symptomatic cases. However, scientists have warned that this approach may underestimate true exposure, particularly when infections are mild or atypical.

To investigate this possibility, the research team analyzed archived blood samples from 176 healthy Nigerian adults who had previously participated in SARS-CoV-2 vaccine studies. The samples included healthcare workers collected in 2021 and community volunteers who were sampled in 2023.

None of the participants had received mpox or smallpox vaccines during adulthood, and none were known to have been exposed to mpox.

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Using a high-resolution multiplex antibody assay, the researchers measured immune responses to six distinct mpox virus antigens — structural components of the virus recognised by the immune system — allowing assessment of both the strength and breadth of antibody responses.

At baseline, 24 participants (14 per cent) showed antibody profiles consistent with residual immunity from historic smallpox vaccination. These responses were largely observed in individuals born before 1980, who were more likely to have been vaccinated during childhood.

Among 153 participants with follow-up samples collected approximately nine months later, five individuals (approximately three per cent of the cohort) showed clear evidence of new immune boosting consistent with recent mpox exposure.

None of these individuals had a recorded mpox diagnosis or reported symptoms compatible with the disease, suggesting that exposure may have occurred without recognized illness.

Lead author Dr. Adam Abdullahi of the University of Cambridge and the Institute of Human Virology Nigeria stated that the findings challenge the conventional assumptions about mpox transmission.

“What we’re seeing is evidence that mpox exposure doesn’t always look like the textbook description. In some people, particularly in settings with partial population immunity, the virus may circulate quietly, leaving immune footprints that routine clinical surveillance will miss,” he said.

The strongest antibody increases were directed against specific viral proteins — B6R, A35R, and M1R — known to be key targets of protective immune responses.

To place the immunological findings in context, the team also analysed more than 100 mpox virus genomes collected in Nigeria over several years. Genomic reconstruction showed slow epidemic growth, frequent transmission dead-ends, and limited clustering, patterns consistent with ongoing transmission constrained by partial immunity.

Importantly, the study found no major differences in immune responses between healthcare workers and the general population, suggesting that exposure is not limited to occupational risk but reflects broader community transmission.

Professor Alash’le Abimiku, Executive Director of the Institute of Human Virology Nigeria, stated that the findings demonstrate the need for broader surveillance strategies.

“Instead of relying solely on reported cases or symptoms, monitoring populations by testing blood samples for antibodies to reveal exposure to the virus will be important for understanding how it is spread and guiding targeted vaccination in our settings,” he said. If we only look for obvious diseases, we will miss part of the picture,” he said.

The researchers stressed that their findings do not suggest a widespread, silent epidemic. Rather, they highlight the complexity of mpox transmission in populations with mixed immunity, noting that the exposure rate reflects the study cohort and should not be interpreted as the population prevalence.