International health officials are working quickly to develop a vaccine against avian influenza in case the virus makes a jump from one that is transmitted between birds and humans to one that can be transferred directly between humans. In the meantime, Canadian health experts say that the bird flu's appearance in a number of Asian countries at the same time suggests that it is more widespread among poultry than believed at first, and they warn that it could move quickly to other regions of the world. As a result, they have recommended that Canada begin stockpiling antiviral drugs as a precautionary measure in case the bird flu does begin to spread among humans, causing a pandemic that some influenza experts say is long overdue in a historical sense. In a moderate pandemic, fully one third of Canada's population could be sick in the first wave of the virus, while a more severe pandemic could lead to one half of the population getting sick.