Study co-author Andrew Noymer, an associate professor of public health at the University of California, Irvine, said deaths from flu pandemics tend to skew younger than those from typical seasonal flu because of "antigenic recycling," the fact that some parts of flu viruses have already made the rounds among older people.
Between 1918 and 1957, all flu viruses in circulation fell into the H1N1 category, so in 2009, older adults had some protection stemming from their prior experience with viruses of this type, Noymer said.
"Excess death rates were highest among 25- to 64-year-olds," Noymer said in a statement.
The researchers used mortality data from the National Center for Health Statistics from January 1959 to December 2010.
The 2009 pandemic was unusual not just for its excess fatalities but for the timing and age distribution of those deaths, Noymer said. October and November of 2009 had the highest flu death rates in those 25-34 since at least 1959, when computerized collection of population data began.
The novelty of the currently emerging H7N9 bird flu strain is noteworthy, because the virus has not been experienced before in anyone's lifetime, Noymer said.
The findings appear in the journal Plos One.