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."