Avian Flu Pandemic Might Be Less Likely
A CDC study released on Monday "suggests that it might be more difficult for the deadly avian flu virus to spark a pandemic than originally feared," the Wall Street Journal reports (McKay, Wall Street Journal, 8/1). For the study, published in the Proceedings of the National Academy of Sciences, CDC researchers simulated what could occur if the H5N1 virus and a human flu virus mixed.
They exchanged some of the H5N1 virus' genes with those from the H3N2 human flu virus. Such a genetic combination has been scientists' "biggest fear," the New York Times reports. The H3N2 virus is "a very contagious strain that causes most of the seasonal flu outbreaks worldwide," according to the Times (Grady, New York Times, 8/1).
Previous studies have indicated that flu epidemics in 1957 and 1968 were caused by the exchange of genes between avian and human viruses. For the study, researchers created three virus variations out of 50 hybrids that were possible, choosing the ones they believed were most likely to cause a pandemic (Chong, Los Angeles Times, 8/1).
The newly created viruses were then tested on ferrets, which are highly susceptible to human flu strains and transmit them easily. Ferrets also can be infected with avian flu but do not spread it easily. The ferrets did not efficiently transmit the hybrid viruses, and they were made less ill by them than they were by the regular human virus (New York Times, 8/1).
The researchers used the only H5N1 strain known to cause infections among humans when the study began in 2002, a 1997 strain from Hong Kong (Los Angeles Times, 8/1).
CDC Director Julie Gerberding said, "These data do not mean that H5N1 cannot convert to be transmissible from one person to another person. They mean that it's probably not a simple process, and more than simple genetic exchanges are necessary" (Manning, USA Today, 8/1).
Gerberding said H5N1 could undergo a more complex genetic exchange with a human flu virus or could mutate on its own -- a process that researchers say occurred to create the virus that caused the 1918 pandemic, which killed millions (Wall Street Journal, 8/1).
Jacqueline Katz, a branch chief in CDC's Influenza Division and one of the study's lead authors, said, "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" (Reichard, CQ HealthBeat, 7/31). Katz added, "However, there is still much we do not know about the molecular changes the virus would need to cause a pandemic" (Berger, Houston Chronicle, 8/1).
CDC scientists and others said H5N1 has undergone significant mutations since 1997 and cautioned that more pathogenic strains have now been found.
Katz said CDC is conducting additional experiments using more recent strains of H5N1 and H3N2 (Wall Street Journal, 8/1).
Frederick Hayden, a medical officer at the World Health Organization, said, "This is encouraging, but doesn't tell us what might be observed if studies were done with more contemporary strains." Hayden added, "The threat is still there and continued work on preparing is absolutely essential" (Young, Atlanta Journal-Constitution, 8/1).
Anna Moscona, an infectious disease specialist at Cornell University's Weill Medical College, said, "The fact that a simple gene swap didn't immediately create a monster virus is reassuring" (Los Angeles Times, 8/1).