A new scoping review by US Centers for Disease Control and Prevention (CDC) researchers reveals that asymptomatic human infections with highly pathogenic avian influenza A(H5N1) do occur, and person-to-person transmission is likely in some cases. The findings, published in JAMA Network Open, underscore the need for better surveillance to inform public health responses.
Key Findings of the CDC Review:
- Global Context: Since 1997, over 1,000 symptomatic human H5N1 infections have been reported globally. The review sought to understand the frequency of infection in people without symptoms, which is vital given ongoing H5N1 outbreaks in animals.
Asymptomatic Cases: Researchers identified 10 reports detailing 18 asymptomatic human H5N1 infections worldwide.
Human-to-Human Spread: Two cases, confirmed through molecular and serologic testing (MSC), were found in adults who were household contacts of symptomatic patients. Crucially, one of these two patients is believed to have been infected through human-to-human spread because they had no exposure to sick or dead poultry.- Exposure Source: Other asymptomatic cases were identified through intensified surveillance of people exposed to infected poultry (Bangladesh, Spain, UK) or through contact tracing (Vietnam, Cambodia).
- Conclusion: The authors stressed that current asymptomatic cases are likely underreported and called for robust data collection on possible asymptomatic A(H5N1) virus infection to strengthen future public health responses.
Experts Note Critical Gaps in Surveillance
A related commentary by Rick Bright and Nicole Lurie highlighted the recent spread of the H5N1 clade 2.3.4.4b virus among dairy cattle since March 2024, with confirmed infections in over 800 herds across at least 16 states.
- Animal Transmission: The virus has spilled over to and is transmitting among other mammals, including cats, dogs, and marine mammals.
- Human Cases: As of mid-2025, approximately 70 human cases have been officially reported in multiple states, most with mild illness, but including at least one death.
Bright and Lurie concluded that the review's findings challenge the traditional view that human H5N1 infection is almost always severe and that human-to-human transmission never occurs. They argued that this points to a "critical gap" in national and global surveillance systems, which tend to focus on detecting symptomatic illness rather than infection itself.
Disclaimer: This content is published only for health awareness and informational purposes. It's not a substitute for your professional medical advice. You must consult a doctor/healthcare professional regarding your specific health concerns.
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