It is exceptional that human fatalities have occurred in China after infection with low pathogenic H7N9 avian influenza virus. So far the only human deaths were reported with H7 (H7N7) and H5 (H5N1) viruses that are highly pathogenic for poultry.
Infections with H9N2 subtype (low pathogenic for poultry) are also known in humans. This H9N2 virus has certain genes that encode for internal proteins, related to genes of the highly pathogenic H5N1 virus.
The highly pathogenic virus H7N9 now found in China does not have features that makes it highly pathogenic for poultry, but does have some genetic characteristics (e.g. mutation in HA that allows for better binding to human receptor, deletion in NA and mutation 627 in polymerase PB2), that also occur in for poultry highly pathogenic viruses which have been detected in human infections with fatal outcome.
In 2003, a veterinarian died in Netherlands after infection with highly pathogenic H7N7. The H7N9-virus that currently causes problems in China, seems to be unrelated to the H7N7 virus from 2003. The H7N9-variant is an entirely different virus that does have the previously mentioned features in common with the H7N7 virus that occurred in 2003 in the Netherlands.
The H7N9 virus is pathogenic for humans, but most likely not for poultry. This will make the circulation of the virus in poultry less noticeable. In Netherlands we have an early warning system and serological monitoring program to detect this kind of viruses at an early stage.