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CDC: Flu Death Forecast 'A Moving Target'

 |  By cclark@healthleadersmedia.com  
   August 27, 2010

Health officials generally guide providers to expect 36,000 deaths annually from influenza. But a federal report shows that the numbers of flu deaths vary widely year-to-year. In the 1986-87 flu season there were just 3,349 deaths. In 2003-04 there were to 48,614.

"Flu deaths are a moving target," said David Shay, MD, medical officer with the Centers for Disease Control and Prevention's influenza division, in a media briefing. Year to year, he said, flu forecasters can only guess what the year will look like based on the pathogenicity of circulating strains.

Shay said the CDC is shifting thinking about influenza deaths as a flat 36,000 annual number. "The CDC is trying to move away from a single number, and rather, give a range over a particular period of time to give a better time text to what flu really means in terms of what it does to the community ."

H3N2 strains cause much more severe disease than other strains. For example, "the average mortality rates for the 22 seasons during which influenza A(H3N2) was a prominent strain were 2.7 times higher than for the nine seasons that it was not," the CDC report said.

During A(H3N2) prominent seasons, the number of deaths was 36,631 compared with 13,544 in seasons when non-A(H3N2) strains were prominent. Why that occurred is unclear.

"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.

Asked how many weeks into each year's influenza season epidemiologists can know what strains are circulating, Shay replied, "that depends. In some seasons, for example, H3N2 will start off big and will be the predominant strain throughout a particular season. In other seasons, we've seen, particularly [in] recent seasons,—2006-2007— we have some influenza, H1N1, [and] some H3N2...at the end of the season."

Length of season, he added, can also affect mortality rates.

The report distinguishes those influenza deaths caused by underlying respiratory and circulatory factors from those caused by pneumonia or influenza.

CDC spokesman Tom Skinner noted in a telephone interview that his agency is concerned that the message—that flu deaths vary year to year depending on the strain—may get lost on providers and the general public who may see the risk as much lower in any given year.

"We don't want this new report to detract from our message that vaccination has been, and will always be – the best way to prevent flu. Even in seasons where we don't have a lot of deaths, we have a lot of people getting sick. Flu imposes a tremendous burden every year on our health system and we know that vaccination is the single most important thing can do."

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