Most reports about seasonal influenza cite an average of about 36,000 deaths in a typical season, but that number is too high and grossly misleading, the Centers for Disease Control and Prevention said Thursday.
The actual average is a little more than 23,000, the agency reported in its Morbidity and Mortality Weekly Report. But even that figure is misleading, the report added, because the numbers have ranged from as low as 3,300 deaths to nearly 50,000 over the last 30 years. The period in the analysis covers up to 2007 and does not include last year's H1N1 influenza pandemic.
"There is no average flu season," lead author Dr. David Shay of the CDC's National Center for Immunization and Respiratory Diseases said in a news conference. The number of deaths "can vary dramatically" from year to year, he said.
The number of deaths in a given year depends on a variety of factors, including how long the flu season lasts, how many people get sick and who gets sick. But by far, the most important factor is the strain of flu that predominates in a given season.
When an H3N2 strain predominates, the number of deaths typically is about 2.7 times higher than in years when an H1N1 strain predominates. Researchers are not sure why that is, but it occurs at least in part because the H3N2 virus mutates more rapidly.
"Even if you have been sick with it in the past, you are more likely to get a subsequent infection," Shay said. It also tends to make more older people ill.
Shay noted that the 36,000 figure that is frequently quoted was an average for the decade of the 1990s, when H3N2 predominated in most years.
During the 30 years covered by the study, nearly 90% of flu-related deaths occurred in people over the age of 65, about 10% in those ages 19 to 64 and about 1% in those younger than 19. One thing that was dismaying about the recent swine flu outbreak: The majority of deaths linked to it occurred in the two younger age groups.
Shay noted that there is no way to tell before a flu season begins — or even a few weeks into the start of the season — which strain will predominate. "Flu really is unpredictable," he said. The best way to protect yourself, he added, is to follow the CDC's recommendation and get vaccinated every year.