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Studies in animals suggest 2009 H1N1 virus may have biological advantage over seasonal influenza

September 01, 2009

Preliminary findings in ferrets suggest that the novel 2009 H1N1 influenza virus may outcompete human seasonal influenza viruses, researchers say. Tests in animals showed that levels of the 2009 H1N1 virus rose more quickly than levels of the seasonal virus strains, and the new virus caused more severe disease. In line with previous findings by other research groups, the University of Maryland researchers also observed that the novel H1N1 virus was transmitted more easily from infected to uninfected ferrets than either of the two seasonal influenza viruses.

The researchers found no evidence that the 2009 H1N1 virus combined with either of two seasonal flu viruses to form new, so-called reassortant viruses. These findings suggest that while 2009 H1N1 virus probably will predominate in the coming flu season, there may not be biological pressure for the new virus to re-combine with other circulating viruses, the researchers say.

The work was done by Daniel Perez, Ph.D., and colleagues from the University of Maryland. The researchers were supported by the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.

"This elegant study, conducted in a useful animal model of human influenza, provides important information about how the 2009 H1N1 influenza virus interacts with other flu virus strains," says NIAID Director Anthony S. Fauci, M.D. "The results suggest that 2009 H1N1 influenza may outcompete seasonal flu virus strains and may be more communicable as well. These new data, while preliminary, underscore the need for vaccinating against both seasonal influenza and the 2009 H1N1 influenza this fall and winter."

When the investigators inoculated ferrets with 2009 H1N1 virus plus either seasonal H1N1 virus or seasonal H3N2 virus, the animals became co-infected with both viruses. However, only the 2009 H1N1 virus was then transmitted from co-infected ferrets to uninfected ferrets; there was no evidence that either of the seasonal flu viruses were transmitted between co-infected and uninfected animals. "The H1N1 pandemic virus has a clear biological advantage over the two main seasonal flu strains and all the makings of a virus fully adapted to humans," says Dr. Perez.

Next, the team conducted experiments to learn whether 2009 H1N1 virus would combine with seasonal flu viruses in co-infected animals to create new reassortant viruses. Some scientists have speculated that reassortant viruses may be more virulent or transmissible than either 2009 H1N1 or seasonal flu viruses alone. The researchers collected virus-containing material from the ferrets' nasal cavities, but found no evidence of reassortment between the 2009 H1N1 and seasonal influenza strains, either in ferrets that were directly infected with both viruses or in ferrets that came in contact with the co-infected animals.

NIH/National Institute of Allergy and Infectious Diseases