The US Centers for Disease Control and Prevention (CDC) said this week that updated estimates of flu deaths in the US in recent decades show that rather than widely cited annual figure of 36,000, which is too high anyway, the estimated numbers have fluctuated from as low as 3,500 to as high as nearly 49,000, depending on which flu viruses have been prominent.
Based on the revised estimates, the annual average is closer to 23,000, significantly lower that the previously cited 36,000. But even that figure is misleading because of the wide fluctuations, said the federal agency.
The CDC published the updated estimates for the period 1976 to 2007, the latest year for which national death certificate reports are available and so it excludes the period of the 2009 H1N1 pandemic, in the 27 August issue of their Morbidity and Mortality Weekly Report (MMWR).
The federal agency explained that the widely cited estimate of 36,000 annual flu deaths was based on an estimating model that only covered the 1990s, which were dominated by influenza A (H3N2) viruses.
"Seasons when influenza A (H3N2) viruses were prominent had 2.7 times more influenza-associated deaths than years when influenza A (H1N1) or B viruses were prominent," said the CDC in a statement.
Experts believe that one reason could be because H3N2 mutates more quickly, thus rendering ineffective any immunity people might have from a previous infection.
Using death certificate reports covering 1976 to 2007, the CDC looked at two categories of underlying cause of death, pneumonia and influenza causes and respiratory and circulatory causes, to estimate the lower and upper bounds for the number of flu-related deaths.
Their estimates show that about 90 per cent of flu-related deaths were of adults aged 65 and over. This is in stark contrast to the recent H1N1 swine flu pandemic which claimed mostly younger lives.
The CDC also analyzed the figures by flu virus type and subtype to see if there were any links between these and the numbers of deaths in a season.
They found that flu-related deaths varied signficantly each year, by type of circulating flu virus, and by age group, and suggest that future summaries should show estimates according to these categories rather just lump them all together into one annual figure.
Because the deaths vary so much in this way from year to year, the overall message is that "there is no average flu season", as Dr David Shay of the CDC's National Center for Immunization and Respiratory Diseases, said in a news conference reported by the Los Angeles Times. Shay is also the lead author of the new report.
Shay said we still have no way of predicting which strains will dominate when a new flu season begins, even in the first few weeks, the picture is unclear.
He urged people to follow the CDC's recommendation and get vaccinated every year as the best way to protect themselves from the flu.
"Estimates of Deaths Associated with Seasonal Influenza - United States, 1976-2007."
MG Thompson, DK Shay, H Zhou, CB Bridges, PY Cheng, E Burns, JS Bresee, NJ Cox: Influenza Division, National Center for Immunization and Respiratory Diseases, CDC.
MMWR, August 27, 2010 / Vol. 59 / No. 33
Source: CDC, LA Times.