The U.S. Centers for Disease Control says it's too soon to say if the new H1N1 flu virus is replacing seasonal flu viruses in the Southern Hemisphere where flu season is beginning or underway.
There have been suggestions the new virus may be crowding out human influenza A strains, something that had been seen in previous pandemics.
But the CDC says flu testing in some countries may be focusing exclusively on finding the novel H1N1 virus, not trying to get a representative sample of all the flu viruses that are circulating.
Dr. Daniel Jernigan of the CDC's flu division in Atlanta says in some countries that have longstanding influenza surveillance programs, a mix of viruses is being seen.
Jernigan says the new pandemic virus predominates in those settings so far, but testing is also turning up some human H1N1 and H3N2 viruses as well as some influenza B.
That means there may be a smorgasbord of flu viruses circulating this fall in the Northern Hemisphere.
"In the past with pandemic strains, there has been a replacement (of influenza A viruses)," Jernigan says.
"But right now we don't have enough information to say that there is a replacement occurring. And at this point we are expecting and at least planning that there will be multiple subtypes that are circulating at the same time this fall."
That's why officials in Canada, the U.S. and other countries where seasonal flu vaccine is used have maintained plans to purchase and administer seasonal flu shots in addition to vaccine to protect against the novel H1N1.
