Background: Highly pathogenic avian influenza A(H5) human infections are a global concern, with many A(H5) human cases detected in Vietnam, including a case in October 2022. Using avian influenza virus surveillance from March 2017‒September 2022, we described the percent of pooled samples that were positive for avian influenza A, A(H5), A(H5N1), A(H5N6), and A(H5N8) viruses in live bird markets in Vietnam. Methods: Monthly at each LBM, 30 poultry oropharyngeal swab specimens and five environmental samples were collected. Samples were pooled in groups of five and tested for influenza A, A(H5), A(H5N1), A(H5N6), and A(H5N8) viruses by real-time reverse-transcription polymerase chain reaction. Trends in the percent of pooled samples that were positive for avian influenza were summarized by LBM characteristics and time and compared to the number of passively detected avian influenza outbreaks using Spearman’s rank correlation. Results: A total of 25,774 pooled samples were collected through active surveillance at 167 LBMs in 24 provinces; 36.9% of pooled samples were positive for influenza A, 3.6% A(H5), 1.9% A(H5N1), 1.1% A(H5N6), and 0.2% A(H5N8). Influenza A(H5) viruses were identified January–December and at least once in 91.7% of sampled provinces. In 246 A(H5) outbreaks in poultry; 20.3% were influenza A(H5N1), 60.2% A(H5N6), and 19.5% A(H5N8); outbreaks did not correlate with active surveillance. Conclusions: In Vietnam, influenza A(H5) viruses were detected by active surveillance in LBMs year-round and in most provinces sampled. In addition to outbreak reporting, active surveillance for A(H5) viruses in settings with high potential for animal-to-human spillover can provide situational awareness.

In South Vietnam, live bird markets (LBMs) are key in the value chain of poultry products and spread of avian influenza virus (AIV) although they may not be the sole factor to determine avian influenza (AI) prevalence in the southern part. Therefore, a risk analysis of AIV spread was conducted by including all possible value chain factors. A cross-sectional study was performed in backyard farms, high-biosecurity farms (bio-farms), LBMs, and poultry delivery stations (PDSs) in the four districts of Vinh Long Province in December 2016 and August 2017. A total of 3 597 swab samples were collected from individual poultry at 101 backyard farms, 50 bio-farms, 58 sellers in LBMs, and 17 traders in PDSs and then investigated for AIV isolation. Concurrently, information related to participants and birds was collected and used to identify the fixed and random effects of factors in AIV infection. A total of 274 birds were positive for virus isolation, with a prevalence of 7.6% (95% confidence interval [CI]: 6.8–8.5) at the individual poultry level, and the adjusted prevalence based on the sampling weight was 7.9% (95% CI: 7.6–8.2). The significantly higher prevalence in PDSs (20.7%) and LBMs (14.2%) compared to backyard farms (3.0%) and bio-farms (0.6%) suggested that PDSs are another hot spot for AIV circulation. The high diversity in the seller and trader population characteristics was revealed using multiple-correspondence analysis to analyze the participants’ demographic factors in LBM and PDS. The mixed-effect logistic regression model revealed that keeping duck at the sampling time and the owner’s older age should be risk factors of AIV infection in PDS. Therefore, functional AI control efforts to monitor the PDS system should be emphasized to minimize AIV circulation risk in Vietnam.