Even if we knew the denominator, the equation would still be meaningless. In areas where a strain of avian flu is endemic, many people will first become immune through their mother's antibodies, and stay immune through repeated exposure to the virus. Outbreaks will naturally be mild and limited, but not because the virus is friendly.
What we need to know are the antibody levels in the population surrounding the outbreaks. If the levels are low, then the virus is mild and less dangerous. If the levels are high, then the virus could well be highly lethal in a pandemic.