HIGHLY pathogenic avian influenza A(H5N1) has caused over 1,000 confirmed human infections since 1997, often resulting in severe disease. However, the frequency of asymptomatic infections has remained unclear, complicating public health assessments. A new scoping review analysed published reports to determine whether humans can carry H5N1 without symptoms.
Review Identifies 18 Asymptomatic H5N1 Cases Worldwide
The review screened 1,567 unique reports, narrowing them to 10 publications describing 18 asymptomatic cases. Of these, two cases were confirmed by both molecular and serologic testing (MSC), while 16 cases had molecular confirmation alone. The MSC cases were adults in Pakistan and Vietnam, identified as household contacts of infected individuals. Exposure sources included both poultry and possible human-to-human transmission. None of these individuals used personal protective equipment.
The 16 MC cases consisted of 14 adults and two children, mainly identified through enhanced surveillance of persons exposed to H5N1-infected poultry across Bangladesh, Spain, the UK, Vietnam, and Cambodia. Several were household contacts of confirmed H5N1 patients. Symptom monitoring methods varied across reports, and most cases would not have been detected without active surveillance.
Implications for Public Health
Asymptomatic H5N1 infections are extremely rare and mostly discovered through targeted investigations rather than routine testing. This finding highlights gaps in understanding the true prevalence of silent infections and the potential risk for unnoticed transmission. The study emphasises the need for prospective surveillance with serial respiratory and serum sampling in high-risk populations, alongside detailed symptom tracking. Such efforts could improve detection and inform future prevention strategies.
While asymptomatic H5N1 infections in humans are possible, they remain infrequent and largely identified through enhanced monitoring of high-risk contacts. Comprehensive data collection is essential to guide public health policies and prevent unnoticed spread during outbreaks.
Reference
Dawood FS et al. Asymptomatic human infections with avian influenza A(H5N1) virus confirmed by molecular and serologic testing: a scoping review. JAMA Netw Open. 2025;8(10):e2540249.
