Researchers, led by Nicholas Savill of the University of Edinburgh in Scotland, used mathematical models to simulate the spread of H5N1 and concluded that "at least 95percent of birds need to be protected to prevent the virus spreading silently. In practice, it is difficult to protect more than 90percent of a flock; protection levels achieved by a vaccine are usually much lower than this." The Food and Agriculture Organization of the United Nations has issued recommendations on the prevention and control of avian influenza in poultry, including the use of vaccination.