Research demonstrated that the H7N9 virus was able to attach both in the upper and in the lower parts of the respiratory tract. Based on what is known from other influenza viruses, attachment to the upper part is important for efficient transmission. The ability to also attach to the lower parts of the respiratory tract, on the other hand, is correlated with severity of disease. This does fit with the high case fatality rate of H7N9. Findings thus confirm that this avian flu virus is a potentially very dangerous virus, should it acquire the other characteristics for efficient transmission. It is suspected, however, that H7N9 lacks sufficient haemagglutinin stability to become efficiently transmissible, and needs to reduce binding to avian receptors.