Objectives: Highly pathogenic avian influenza (HPAI) A(H5N1) has caused sporadic human infections worldwide since the first human case was reported in Hong Kong in 1997. Poultry farm workers with direct contact with infected poultry are considered at increased risk for avian influenza infection. This study evaluated evidence of human infection through serologic testing among poultry farm workers epidemiologically linked to A(H5N1) outbreaks in the Republic of Korea (ROK) during the 2024-2025 season.

Methods: Acute- and convalescent-phase sera were collected through local public health centers from 41 poultry farm workers associated with HPAI A(H5N1) outbreaks in the ROK during the 2024-2025 season. Moreover, antibody detection was performed using a microneutralization test. According to the "Response Guidelines for Human Infection with Animal Influenza, a Class 1 Infectious Disease," seropositivity was defined as ≥4-fold increase in neutralizing antibody titers between convalescent- and acute-phase sera.

Results: We analyzed 41 paired serum samples collected from Sejong, Gyeongbuk, Chungnam, Jeonnam, Gyeonggi, Incheon, Jeonbuk, and Gyeongnam provinces. Neutralizing antibody titers were ≤10 in all samples, and no ≥4-fold increase in antibody titers was detected in any participant.

Conclusions: In this investigation, no serologic evidence of HPAI A(H5N1) infection was identified among exposed poultry farm workers during the 2024-2025 outbreak period. However, considering the ongoing reports of human infections internationally and the expanding host range of the virus, including mammalian infections, continued active surveillance and periodic serologic monitoring of occupationally exposed high-risk populations are warranted.