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It is possible that during replication in birds or humans, the H7N9 viruses might randomly acquire a mutation that allows for transmission. In the right place at the right time, such a virus could spread through the human population. Alternatively, such a transmission-facilitating mutation might interfere with the overall fitness of the virus, thereby preventing it from spreading. I favor the latter hypothesis because the H7N9 viruses have been transmitting since at least February 2013; they have undergone many replication cycles without such a mutation arising. If the virus has not entered wild birds, culling poultry could eradicate it from China – assuming that it has not gone elsewhere.