Unnoticed Entry, Noticed Transmission: Epidemiological Insights into China's First Clade Ib Monkeypox Virus Cluster.

BACKGROUND

The mpox outbreak caused by Clade Ib monkeypox virus (MPXV) spread rapidly in Central and East Africa and led the World Health Organization to declare the outbreak a public health emergency of international concern (PHEIC). In response, China implemented enhanced surveillance measures; nevertheless, an ostensibly asymptomatic returning traveler with repeatedly negative screening tests initiated China's first documented Clade Ib mpox transmission cluster.

METHODS

We conducted an epidemiological, clinical, environmental, and genomic investigation to characterize the transmission pattern and inform public health response. Real-time polymerase chain reaction (RT-PCR), serological testing, and whole-genome sequencing were performed on human and environmental samples to confirm infection and assess viral contamination.

RESULTS

Four epidemiologically linked individuals were identified, including 1 imported index case, 2 first-generation confirmed cases resulting from unprotected heterosexual contact with the index case, and 1 suspected second-generation case associated with intimate contact (overnight cosleeping). No fever was documented during clinical assessment or follow-up; 2 cases presented with rash, and 1 had inguinal lymphadenopathy. Whole-genome sequencing confirmed infection with Clade Ib MPXV. All 158 throat swabs collected from general (nonsexual) contacts tested negative. Among 314 environmental samples, 25 of 62 samples collected from case residences were MPXV qPCR positive, whereas all 252 samples from public settings were negative.

CONCLUSIONS

These findings demonstrate that Clade Ib mpox transmission can occur from an apparently asymptomatic returning traveler despite repeated negative entry screening, underscoring the limitations of symptom-based or entry-point screening alone and the need for enhanced, risk-based surveillance, diagnostic testing, and targeted prevention strategies.