Asymptomatic human avian influenza A(H5N1) infections occur, and person-to-person transmission is likely in some settings, reveals a scoping review by US Centers for Disease Control and Prevention (CDC) researchers.

The study, published yesterday in JAMA Network Open, involved a search for studies on confirmed highly pathogenic H5N1 infection published through August 25, 2025.

The team identified 10 reports of 18 asymptomatic human H5N1 infection, including 2 with molecular and serologic confirmation (MSC) and 16 with molecular confirmation alone (MC).

High viral levels have been found in raw milk, and live virus has been detected in mammary tissue and milking equipment.

“Spillover into, and transmission among, other mammals has been documented, including cats, dogs, mice, wild carnivores, marine mammals, and swine,” they wrote. “Human cases have been confirmed in multiple states, and as of mid-2025, approximately 70 cases have been officially reported, most with mild illness but including at least 1 death.”

Bright and Lurie said the findings challenge the traditional perception that human H5N1 infections are almost always symptomatic and severe and that no human-to-human H5N1 spread has occurred.

“It also highlights a critical gap in many national and global surveillance systems, which tend to focus on the detection of symptomatic illness rather than infection, and underscores the importance of carefully examining clusters where limited transmission may occur,” they concluded.