Post-vaccination surveillance during the first year of duck vaccination against highly pathogenic avian influenza in France: Adherence to protocols and first outcomes in terms of viral circulation.

In response to recurrent epizootics of highly pathogenic avian influenza (HPAI), France launched a national vaccination programme targeting ducks in October 2023. It was completed by a three-pillar post-vaccination surveillance system, comprising enhanced passive surveillance, active virological testing, and end-point serological testing. Using national-level data from mandatory reporting systems, this study (i) evaluates the implementation of the post-vaccination surveillance system during its first year, and (ii) presents initial results regarding the detection of avian influenza virus (AIV) circulation in vaccinated flocks through this surveillance programme. A total of 18,881 enhanced passive surveillance operations were recorded across 1604 establishments, representing a coverage rate of 55.2%. In addition, 17,951 active surveillance operations were conducted in 2011 establishments, with virological testing performed in 89.0% of targeted sites and serological testing in 83.4%. All surveillance items considered, 92.3% of the establishments housing vaccinated ducks carried out at least one surveillance operation over the studied period. The three surveillance components proved complementary: AIV prevalence at the establishment level was estimated at 6.8%, 7.3% and 13.7% through passive surveillance, active virological testing, and end-point serological testing, respectively, with only low pathogenic AIV detected. These findings underscore (i) the unprecedented scale of AIV surveillance within the French duck farming sector, (ii) the importance of evaluating stakeholders' adherence as a prerequisite for assessing surveillance effectiveness; (iii) the interest of monitoring AIV circulation to detect the potential emergence of antigenically modified virus strains and to inform timely adjustments to control strategies.