New way to assess pandemic potential of influenza viruses

Aug 1 2006

Study provides first assessment of the risk of an H5N1 pandemic strain emerging from the combining of avian and human influenza viruses.

Researchers at the Centers for Disease Control and Prevention (CDC) have developed a new research method that may help identify the types of genetic changes necessary for the avian influenza virus (H5N1) to be more easily transmitted among people.

After developing the research method, CDC scientists used it to investigate the ability of a lab-engineered combination of the avian influenza virus and a more common human virus to spread in lab animals. Efficient and sustained human-to-human transmission is the remaining property that H5N1 avian influenza viruses do not yet have that is needed to cause a pandemic.

In this series of experiments, published in the July 31 issue of the journal Proceedings of the National Academy of Sciences, genes from a human H3N2 influenza virus were added to genes from an H5N1 avian influenza virus to create new hybrid viruses. The new viruses were tested in ferrets because their susceptibility to flu viruses is similar to that of humans. The animals were then placed in close proximity, to see if infected ferrets passed the new virus to uninfected animals and whether they transmitted it more easily than the original H5N1 virus.

In this model, human H3N2 viruses transmitted efficiently between the ferrets, but avian H5N1 viruses did not. When the hybrid viruses were tested it was found that these viruses also did not pass easily between ferrets.

"This important science has established a new research method to help us learn more—in advance—about the genetic changes that enable new influenza viruses to spread efficiently and in a continuous manner among people," said CDC Director Dr. Julie Gerberding. "H5N1 viruses continue to spread among birds worldwide and their genetic properties are constantly changing. There is an urgent need to better understand how these viruses could acquire the ability to spread efficiently between people. This research increases our knowledge, and may enable us to more quickly identify H5N1 viruses and other influenza viruses that have the potential to cause a pandemic."

The avian flu H5N1 viruses currently circulating already possess two of the three characteristics that scientists believe are needed to cause a pandemic. The first characteristic is being a new virus to which humans have little or no immunity. The second characteristic is the ability to infect people and cause illness. The CDC studies were designed to help researchers learn what genetic changes would be needed for the virus to gain the remaining trait necessary to cause a pandemic: the ability to spread easily from person to person in a sustained manner within the population.

To do this, Dr. Taronna Maines and her CDC colleagues designed and tested a research method that involved three elements: ferrets; a caging system that enabled researchers to put healthy and infected animals in close proximity; and reverse genetics, a tool for combining the genes from human and avian influenza viruses. Several H5N1 viruses were used in the research along with a H3N2 strain, a common human influenza virus that circulates nearly every year. Infected ferrets were either placed in the same cage with uninfected ferrets to test transmissibility by close contact or in adjacent cages with perforated walls to test spread of the virus by respiratory droplets.

The studies showed that the H3N2 virus passed easily by droplets but the H5N1 virus did not, reflecting what is seen with these viruses in humans.

Researchers then swapped genes from a 1997 H5N1 avian flu virus with genes from an H3N2 virus, in a process called reassortment. When tested using the ferret model, these "hybrid" viruses (viruses that contained both avian H5N1 and human H3N2 influenza virus genes) did not pass easily between ferrets and, in fact, caused less severe disease than the original H5N1 virus. The reassortment work was designed to mirror the phenomenon that occurs in nature when two flu viruses combine to form a new virus, a process that led to the 1957 and 1968 pandemics. It is still unknown whether the H5N1 virus could reassort with a human influenza virus in nature.

In a final study, CDC researchers passed a hybrid virus through a series of ferrets to see if the virus would accumulate genetic changes necessary to transmit more easily. The researchers found the process introduced only one genetic change in the virus but didn't enhance its transmissibility.

Although the findings apply only to the specific viruses used in the study, the research suggests that significant genetic changes in the H5N1 virus would likely be needed to create a strain that could cause a pandemic. Future CDC studies will examine whether combining genes from H3N2 strains with more recent H5N1 strains makes the new virus more easily transmissible.

"This study provides for the first time an assessment of the risk of an H5N1 pandemic strain emerging through reassortment with a human influenza virus. However, there is still much we do not know about the molecular changes the virus would need to cause a pandemic," said Dr. Jackie Katz, a branch chief in CDC's Influenza Division and one of the lead researchers on the paper. "Influenza viruses are constantly changing so we need to be vigilant and continue our work using this research method to better understand if there are other possible virus combinations or emerging changes in the H5N1 viruses that would increase the risk of a pandemic strain emerging."

Most influenza experts believe another pandemic will occur, but it is impossible to predict which strain will emerge as the next pandemic strain, when it will occur or how severe it will be. As of late July, H5N1 had caused more than 230 cases of disease in humans worldwide and is widespread in bird populations in Asia, Africa and Europe. However, the virus has only rarely passed between humans and does not currently transmit easily from human to human. H5N1 avian viruses have not been found in the United States in either birds or humans.

The research was done in collaboration with Centro Nacional de Biotecnologia, Madrid, Spain; National Institute of Hygiene and Epidemiology, Hanoi, Vietnam; and the Center for Biomedical and Pharmaceutical Research and Development, Ministry of Health, Jakarta, Indonesia, which provided reagents and viruses for the study. All laboratory work was conducted at CDC under stringent safety precautions.