But as this figure from the CDC shows, the current season has been dominated by H3N2.
That includes the 2007-08 and 2003-04 seasons, which also saw a predominant H3N2 strain.
And this year we had a big spike in deaths due to flu, all because of H3N2.
An H3N2 flu strain has been the most dominant and has been especially hard on the elderly.
H3N2 viruses are a subgroup of influenza A viruses, and they are known to adapt in humans.
Of the H3N2 viruses tested at that point, 99.4% matched the type that the vaccine protects against.
H3N2 viruses are a subgroup of influenza A viruses and they are known to adapt in humans, Bresee said.
After Spanish flu in 1918 and Asian flu in 1957, there was another pandemic, Hong Kong flu (H3N2) in 1968.
According to the CDC, H3N2 flu viruses are common among pigs.
In a laborious series of steps, Bucher's team guides the changes by adding antibodies that eliminate the surface proteins of the H3N2 virus.
The 2012-13 flu season is said to have started earlier than usual, with many cases of the H3N2 strain, which can be severe.
This year's North American vaccine is made from three viruses: two types of influenza A virus (H3N2 and H1N1) and an influenza B virus.
Early data from the CDC makes it clear that both strains are still around, with H3N2 showing up somewhat more frequently so far this fall.
The type of flu that is going around is called H3N2, which is often linked to more serious disease compared to other flu varieties, Fauci said.
In addition, while this year's vaccine was considered a good match for the most common circulating flu viruses, it only provided 47% protection against H3N2, the main virus.
"One possible explanation for this is that some older people did not mount an effective immune response to the H3N2 component of this season's vaccine, " the report said.
The strain H3N2 of the flu virus is causing many of the cases, reports suggest, and it is known to cause severe illness, especially among the elderly.
So Bresee said preliminary steps have been taken to develop an H3N2 vaccine -- part of the overall pandemic preparedness planning of the CDC and other health agencies.
Bresee said last week that preliminary steps have been taken to develop an H3N2 vaccine, part of the overall pandemic preparedness planning of the CDC and other health agencies.
What makes this new version of the H3N2 flu virus different is that it has picked up a gene from the novel H1N1 flu virus that became a pandemic three years ago.
As of January 5, the CDC said Friday, the predominant virus in the United States was an influenza A (H3N2) virus that matched well with the H3N2 virus represented in the vaccine.
Bresee said he is not equating this new H3N2 flu with the 2009 flu, but a new H3N2 vaccine is in the early stages of development and clinical trials are expected later this year.
As for why this winter's flu season seems especially bad, Dr. Marr believes it is because of the particular strain of the virus, H3N2, which causes "stronger" symptoms, as well as an early start to the season.
People may get more complications from this particular strain of H3N2, "which may make them ill for a longer period of time, " Dr. Michael Jhung, medical epidemiologist in the influenza division at CDC, told CNN's Mary Snow.
Somewhere along the line, H3N2 and H1N1 viruses were present in a mammal at the same time, and the "matrix-gene" (or m-gene) from the H1N1 pandemic virus was picked up by the H3N2 swine flu, creating a new or variant version of H3N2.
Somewhere along the line, H3N2 and H1N1 viruses were present in a mammal at the same time and the "matrix-gene" (or m-gene) from the H1N1 pandemic virus was picked up by the H3N2 swine flu, thus creating a new or variant version of H3N2.
应用推荐