Although officially all cases of H7N9 have resulted from bird to human contact, there are strong indications that this assertion is false:
1. There is no virological link between between birds and affected humans.
2. Mutations have been found in humans but not birds which indicate that the virus is adapting to a human host rather than an avian host.
3. The rapid spread of cases in China is consistent with human to human dissemination but not bird to human transmission.
At the least, the precautionary principle should be invoked given the available evidence and the apparent lethality of the virus. Insisting on a high bar before considering the possibility that this virus could cause a pandemic would be foolhardy.
Is it still possible to prevent a pandemic from starting? Maybe. Viruses vary in their infection efficiency. Models suggest that viruses with lower R0′s, say under 1.6, would be relatively easy to stop from causing pandemics. I am somewhat dubious of these models. But it does make sense to intervene at the earliest possible timepoint in viral spread. H7N9 may still be in the process of adapting to the human host. Hence, it may transmit at a slow rate now. However, once it is fully adapted, the spread may accelerate. The only way to prevent adaptation is to stop the spread among humans. With each passage from one human to another, selective pressures will force the virus to adapt more completely to humans increasing the efficiency with which it is spread.
What can be done? Increase surveillance: we need a massive effort to test large numbers of people wherever cases have been reported as soon as possible; Social distancing: closing schools, theaters, shopping centers and other venues where people meet; Tamiflu blanket: give this antiviral to all contacts of infected patients; Travel restrictions: limit travel from and to affected areas.
The Chinese government and its hand-picked Director of the WHO may hesitate to take these actions for fear of the economic costs. But the consequences of not acting now, when there may still be time, could be far higher.
Longini et al. 2005 Containing Pandemic Influenza at the Source. Science. 309: 1083-1087