16 people have been reported infected with H7N9 in China thus far. 6 of them have died. This represents a case fatality rate (CFR) of almost 40%. This would place H7N9 among the most lethal diseases known to man. However, there have been rumors that there have been many more cases that have not been reported. If we suppose that many cases that have not resulted in death have not been reported then the CFR would be lower.
Most flu viruses have a CFR of less than 0.1%. If H7N9 was no more lethal than “regular” flu, there would have to be 6,000 unreported cases. There is no way so many cases could come from birds in such a short amount of time. Hence, if H7N9 is no more lethal than regular flu, the virus is already spreading human to human in a sustained way.
The Spanish flu of 1918 had a CFR of 2%. If H7N9 is as bad as the worst flu pandemic in human history, then there would already have to be 300 cases. Again, it is unlikely that so many cases could come entirely from birds in such a short period of time. At least some human to human spread would have to be occurring.
R0 describes the efficiency with which flu spreads. How many other people does each infected person infect? This number can be misleading because in the real world many people with flu do not infect anyone else but a few infect many, so-called superspreaders. One H7N9 superspreader in a Shanghai hotel or airport could seed the virus in people who will take it with them to cities around the world.
We don’t know how efficiently H7N9 is spreading in the human population or how lethal it is. However, we do know that either it is much more lethal than regular flu OR it is already spreading sustainably within the human population. Either way, efforts to contain it should be considered an international top priority.