STEVE INSKEEP, host:
Now most years, the common flu viruses change a little bit and require vaccine makers to tweak their annual shots. But three times in the past century, substantially new flu viruses have emerged. Biologists say there's a good chance we're now in the midst of another one those big shifts.
NPR's Richard Harris has more.
RICHARD HARRIS: Scientists who study the influenza virus are always being kept on their toes, because the virus strains that circulate around the globe are constantly mutating from one year to the next
Dr. WILLIAM SCHAFFNER (Vanderbilt University Medical School): Think of those strains that vary from year to year as though they were cousins in the same family. They're variations on a theme.
HARRIS: But William Schaffner at Vanderbilt's Medical School says the emergence of this new virus is different. This virus, which originated partly in pigs, could be a game changer.
Dr. SCHAFFNER: This one is completely new. This is a stranger in town. We've never seen one like this before.
HARRIS: It's not merely a case of the usual and gradual evolution, which biologists call genetic drift.
Dr. SCHAFFNER: This is a shift virus. This is one that's quite different than what's come before.
HARRIS: Scientists have seen these shifts before. The most famous shift was the virus that appeared in 1917 and caused the deadly flu pandemic of 1918. That virus then became the dominant variety of flu for several decades. Then in 1957, it was displaced by another shift virus. And Stephen Morse of the Columbia University School of Public Health says there was yet another shift in 1968.
Dr. STEPHEN MORSE (School of Public Health, Columbia University): There does seem to be a rule of thumb about this, which is that every pandemic in the 20th century essentially established the variants that would become the circulating seasonal influenzas until the next pandemic came along to displace them.
HARRIS: Each shift led to a stronger-than-usual flu season, but each one also calmed down after a year or two, once the population got exposed to the new viruses or got vaccinated. Morse says the question now is whether the new H1N1 Swine Virus will keep moving from person to person efficiently.
Dr. MORSE: If it continues like that, we'll expect to see this virus chugging along and probably the next seasonal influenza will be a descendent of this one.
HARRIS: The question then is how nasty the virus will end up being. John Oxford at St. Bart's and the Royal London Hospital says there's some reason for cautious optimism.
Professor JOHN OXFORD (St. Bartholomew's and Royal London Hospital): In one sense, it's one of the mildest shifts because most people on the planet have got some memory, have come across H1N1 viruses since 1978.
HARRIS: That's right. Even though health officials are calling this new virus H1N1, that's also the type of virus that's in wide circulation today, and it has an interesting history. It was the dominant flu virus through the 1920s, '30s and '40s. Oxford says it disappeared in 1957 when it was displaced by another flu virus, but then a strain of H1N1 suddenly reappeared in 1977.
Prof. OXFORD: Now where could it have come from? And we reckon now, in retrospect, it was probably released accidentally from a laboratory, probably in northern China or just across the border in Russia, because they were - we were - everyone was experimenting with those viruses at the time in the lab.
HARRIS: It was nothing malicious, Oxford believes, just some flu vaccine research that broke out of containment. The descendents of this virus are still circulating. Professor Oxford notes that most people who have encountered the newly emerged H1N1 virus seem to have developed only mild disease, and he speculates that's because we have all been exposed to a distant cousin: the H1N1 virus that emerged in the 1970's.
Prof. OXFORD: That escaped virus perhaps will now provide some benefit in the face of this pig thing.
HARRIS: Again, this is well-informed speculation, not iron-clad assurance. And there is a less reassuring lesson from the previous big shifts in flu viruses. They caused mild disease when they first appeared in the spring, but they all caused big flu seasons when they returned in the fall as the new dominant virus. That's one reason that health officials are taking the new virus very seriously.
Richard Harris, NPR News.
INSKEEP: And if you're still wondering what to call this new virus, you're not the only one. And you can join the conversation at our blog, Flu Shots, at npr.org. Transcript provided by NPR, Copyright NPR.
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