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The CDC has a fascinating graphic showing the relative prevalence of the three major types (H1N1, H3N2, Influenza B) of seasonal influenza in circulation this year, broken down by regions of the country.
As you can see, while the H1N1 virus is the dominant flu strain across the United States this year (as opposed to Europe where the H3N2 is most common), the level of H3N2 varies from a low of less than 3% to a high of over 30%, depending on the region of the country.
The percentage of H1N1 remains fairly consistent (range 60%-81%), with the B virus and H3N2 jockeying for runner up.
In Texas, Arkansas, Louisiana, and Oklahoma the B virus seems to have won that battle, while in Arizona, Colorado, Idaho, Montana, Nevada, Utah and Wyoming the H3N2 virus takes second place.
Here are the 10 regions broken down in tabular form.
You might be wondering why any of this matters?
For many of us, it really doesn't.
If we get the flu, we take to our beds, drink plenty of fluids, take a fever reducer/pain reliever, and in a few days or a week we are back on our feet again.
But for the elderly or for people with pre-existing conditions that might increase their risks if infected, doctors may wish to prescribe an antiviral medication.
And this year, with the rise in resistance of the H1N1 virus to oseltamivir, that isn't particularly easy.
For all practical purposes, oseltamivir (Tamiflu) is now useless for treating the H1N1 virus - which accounts for about 68% of all flu's in the United States this year.
This has been a rapidly developing transition. A little over a year ago, resistance to Tamiflu was practically unheard of.
Oseltamivir is still effective, however, for the B viruses, and for H3N2 - which account for a little over 30% of this year's infections.
Luckily, an older class of antivirals - Rimantadine and Amantadine - are both very effective for the H1N1 virus.
But they don't work on the H3N2 viruses.
That strain acquired resistance to the adamantane class of drugs about 4 years ago.
And the adamantane's don't work for the B viruses, either.
Confused yet?
All of this resistance has led to new recommendations from the CDC on the use of antivirals this year. (See Interim Antiviral Guidance for the 2008-09 season December 19, 2008)
For now, Zanamivir (Relenza) remains effective against the H1N1 virus, and is now recommended over Tamiflu. It is, however, an inhaled powder, and can be difficult for some people to take.
As an alternative, a combination antiviral cocktail of oseltamivir and Rimantadine can be considered in some cases.
There are rapid influenza tests that some doctors use in their office to determine if someone has influenza. Some (but not all) can differentiate between an Influenza A virus and an Influenza B virus.
But they don't discern between H1N1 and H3N2.
Below is a decision chart, designed to help the clinician decide what antiviral(s) are appropriate given the locally prevalent influenza strain, and rapid test results.
CDC asks that providers refer to the FluView Weekly U.S. Influenza Surveillance report throughout the season and contact their state public health department for information on local flu activity.
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