In the study, conducted on ferrets, levels of the H1N1 swine flu virus rose more quickly than seasonal flu and the new virus caused more severe disease.
The H1N1 virus was also transmitted more easily. Dr. Daniel Perez, who headed the research with the support of the National Institute of Allergy and Infectious Diseases, said the results indicate that the vast majority of the influenza cases seen this winter may be the 2009 H1N1 virus.
"The major advantage it has over seasonal flu is transmission," said Perez. "It replicates faster and spreads faster."
The study also found that when the ferrets were simultaneously infected with the H1N1 strain and seasonal influenza, the viruses did not recombine to form a new virus.
"The worst fear has been that it might combine with seasonal strains," said Perez. "By combining with the seasonal strain the thought was maybe this virus will become more virulent."
Dr. Andrew Pekosz, an associate professor in microbiology and immunology at Johns Hopkins University, studies influenza. He said the 2009 H1N1 virus is lacking several of the known genetic factors for virulence present in the 1918 influenza, the avian H5N1 influenza and the seasonal flu, which is why there is so much concern about it combining with other flu strains.
"If we look at it based on what we know about influenza this virus shouldn't be doing very well," said Pekosz. However, he said it is possible "this virus has some unknown changes that allows it to compete better."
The possibility that the H1N1 strain could recombine with the deadly, but much less transmissible, H5N1 avian flu is a continued concern in the global health community. Perez said the two viruses have not yet been tested together, but that research will begin soon.
Despite the findings that the H1N1 virus is slightly more virulent than the seasonal flu, Perez said the more severe illness seen in humans from H1N1 can probably be attributed to the fact that it is a new virus for which most of the population has no immunity.
"The big question is will this pandemic strain have what is needed to be maintained in the population after the pandemic wave is gone, after most people in the population have developed immunity," Perez said.
This may take several years, and several flu seasons to determine. The southern hemisphere has now experienced its first flu season with the 2009 H1N1 influenza present. The flu season there hit earlier than usual and it hit its peak much faster than normally.
There were also more cases of flu and more hospitalizations, a pattern that will be repeated in the northern hemisphere, said Pekosz.
"People are saying it's just like seasonal flu," he said. "It is in respect to how it is causing disease but the vast majority of the population will be susceptible to infection with this virus, where as in a normal influenza year much of the population has some immunity."
An average of 36,000 people die of the flu every year in the United States, according to the Centers for Disease Control and Prevention. With H1N1 circulating, there may not be an increase in the percentage of deadly flu cases but the volume of cases is likely to be higher and thus the number of deaths. A White House report released in August predicted that the virus could infect up to half of the U.S. population and cause as many as 90,000 deaths.
Pekosz said that is why it is so crucial that those most at risk be vaccinated as soon as it becomes available.
"We have already been seeing a tick up in the number of h1n1 cases on college campuses," he said. "It's probably going to continue to increase up to the time that a vaccine is available."
While the very young and very old are normally most at risk for severe illness from influenza, the 2009 H1N1 appears to hit children and people in the 17 to 40 age group harder and has spared the elderly most severe illness. The biological reason for that has yet to be determined, though there is a small population over the age of 50 that has some immunity to the disease.
There is speculation people over age 50 were exposed to an H1N1 virus circulation in the human population before 1957, and have thus built antibodies that recognize the 2009 virus.