On May 22, 2009, WHO chief Dr. Margaret Chan said that the virus must be closely monitored in the southern hemisphere, as it could mix with ordinary seasonal influenza and change in unpredictable ways. "In cases where the H1N1 virus is widespread and circulating within the general community, countries must expect to see more cases of severe and fatal infections," she said. "[152] Experts writing in the July New England Journal of Medicine note that "historically, pandemic viruses have evolved between seasons, and the current strain may become more severe or transmissible in the coming months." They therefore stress that "international cooperation will be crucial" to engage in proper surveillance to help monitor changes in the virus's behavior, which will aid in both "vaccine targeting" and interpreting illness patterns in the fall of 2009.[111]
Other experts are also concerned that the new virus strain could mutate over the coming months. Guan Yi, a leading virologist from the University of Hong Kong, for instance, described the new H1N1 influenza virus as "very unstable", meaning it could mix and swap genetic material when exposed to other viruses. During an interview he said "Both H1N1 and H5N1 are unstable so the chances of them exchanging genetic material are higher, whereas a stable (seasonal flu) virus is less likely to take on genetic material." The H5N1 virus is mostly limited to birds, but in rare cases when it infects humans it has a mortality rate of between 60% to 70%.[153] Experts worry about the emergence of a hybrid of the more virulent Asian-lineage HPAI (highly pathogenic avian influenza) A/H5N1 strain (media labeled "bird flu") with more human-transmissible Influenza A strains such as this novel 2009 swine-origin A/H1N1 strain (media labeled "swine flu"), especially since the H5N1 strain is and has been for years endemic in birds in countries like China, Indonesia, Vietnam and Egypt.[153][154] (See the suite of H5N1 articles for details.)
Nor had federal health officials in the U.S. dismissed the possibility that the worst was yet to come. "Far from it," Ann Schuchat of the CDC says, noting that the horrific 1918 flu epidemic, which killed hundreds of thousands in the United States alone, was preceded by a mild "herald" wave of cases in the spring, followed by devastating waves of illness in the autumn. "That 1918 experience is in our minds," she said.[155]
As of late July, U.S. health officials said that the swine flu "isn't yet mutating to become more dangerous," but they are closely tracking that as the virus continues to circle the globe.
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