If a single introduction of a pandemic-capable strain is likely to happen, then multiple introductions are also likely.

We can't predict what a virus we've never seen will do.

Right now, we can make roughly 60 million doses domestically of a normal flu vaccine in a normal year, ... But according to recent research, the total amount of antigen required to immunize a person against H5N1 is four times as much as the total amount in a normal flu shot. In a pandemic, nearly everyone would need flu vaccines, but with the current version, we could immunize only 15 million Americans, just over 5% of the country.

In normal flu years, most of us have immunity, either from the vaccine or from having the flu in previous years, ... But in pandemics, we have no prior immunity, and it's just like being hit with a completely new disease that we've never built up any ability to fight. That's why the mortality tends to be high even in the age groups that don't usually get very sick from flu.

In order to become a pandemic, it will have to change ... And we don't know what that changed strain will do.

The earlier work focused on the potential as they saw it and the difficulties of doing it once. And we're making the suggestion that doing it once might well not be enough.