"One of the factors may be that the virus has a tendency to genetically change more quickly than the other two viruses," Shay said. "So it changes more rapidly, and therefore, even if you've been sick with it in the past, you're more likely if it changes quicker to get a subsequent influenza infection."
The wide range is reported in the CDC's Morbidity and Mortality Weekly Report and covers 31 seasons from 1976-77 to 2006-07. The report looked only at deaths from flu each year, not illness, hospitalizations, lost productivity, or time off work. Neither did it examine costs.
Shay said that no other vaccine-preventable disease has this range of variability in the number of people it sickens and kills.
Some of the added cases in recent years, especially since childhood influenza cases became reportable in 2004, might be due to better reporting and better diagnostics, Shay acknowledged.
However, with respect to mortality, most of the influenza deaths occur in people age 65 or over.