THURSDAY, Aug. 26, 2010 (HealthDay News) -- Over the past 30 years, the number of annual flu-related deaths in the United States has ranged from a low of about 3,300 to a high of about 49,000.
This is a revision of the static estimate of 36,000 annual deaths that has been reported consistently for years by the U.S. Centers for Disease Control and Prevention.
The update simply underscores the moving-target nature of influenza -- that there is no average flu season, CDC officials said Thursday.
"We want to point out the incredible variability of influenza seasons," said Dr. David Shay, lead author of a report appearing in the Aug. 27 issue of the Morbidity and Mortality Weekly Report, a publication of the CDC.
"Because of the variability of the flu, we're moving from an average to a range," added Shay, who spoke at a morning news conference.
The new numbers reflect both an expanded time period and different strains of the flu in different years.
"There are at least four factors that affect mortality in any given year: the specific influenza strain, the length of the season, how many people get sick and who gets sick -- whether it's hitting younger or older people differentially," explained Shay, who is a medical officer with the CDC's National Center for Immunization and Respiratory Diseases.
The new range of numbers takes into account deaths from the 1976-77 flu season through the 2006-07 season, while the old number only looked at the 1990s, when influenza A (H3N2) was a dominant strain.
The lowest mortality figure cited Thursday occurred during the 1986-87 flu season, while the high came during the 2003-04 season. This translates to 1.4 to 16.7 deaths per 100,000 persons.
When the H3N2 strain of flu was predominant, as it was during the 1990s, there were almost three times more deaths than in seasons when other strains dominated.
"On average, H3N2 is more severe than H1N1 and 'B' influenza seasons," Shay said.
In the 31 seasons reflected in the new numbers, about 90 percent of deaths occurred in adults 65 and older, he said.
This contrasts with what has been seen since mid-2009 with the H1N1 "swine" flu circulating, where younger people and children seem most affected.
And no one knows what will happen this coming flu season.
"We have a vaccination [for H1N1] this year so we may not see much," said Dr. Len Horovitz, a pulmonary specialist with Lenox Hill Hospital in New York City. "The crystal ball is a little murky."
Added Shay: "Flu deaths are a moving target. The [new] numbers confirm that influenza has a substantial burden on mortality, but that burden can be quite varied."
The CDC has more on the seasonal flu.