The United States is prohibiting imports of poultry from countries affected by highly pathogenic avian influenza (HPAI) or from flocks that have been vaccinated against the disease. The US Department of Agriculture (USDA) claims that vaccinated birds may not show signs of infection, making it difficult to detect the presence of the virus in a flock. The US is particularly concerned about France's decision to vaccinate, as it poses a risk of introducing HPAI into the country.