CDC Warns That New Swine Flu Strain Has 'Pandemic Potential'

The CDC says the strain has infected at least 18 Americans since 2010.

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A paper published Tuesday by scientists at the Centers for Disease Control suggests a new swine flu virus has the potential to cause an outbreak.

The A(H3N2)v swine flu strain that has infected at least 18 Americans since Sept. 2010 has shown the potential for human-to-human transmission. According to the paper, which was published in the Proceedings of the National Academy of Sciences, the H3N2 strains "resemble viruses with pandemic potential." Terrence Tumpey, one of the authors of the study, says the current seasonal flu vaccine won't protect against this swine flu strain, although he says the CDC is working on creating a vaccine for swine flu variants such as the one he studied.

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In November, the CDC suggested that "limited human-to-human transmission" of H3N2 had occurred in Iowa, but the most recent findings show that the virus is more easily transmissible than originally thought, leading the authors to warn that "swine-origin H3N2 viruses have the potential to cause additional human disease." Since August, people in at least five states (Indiana, Iowa, Maine, Pennsylvania and West Virginia) have caught the strain.

The paper warns that people born after the mid-1990s may be "particularly susceptible to infection" because of a virus that circulated in the early part of that decade that may have given some people a low level of protection.

The virus was shown to be highly transmissible from ferret to ferret, an animal which has long been used to explore the possibility of human-to-human transmission of viruses.

"The use of the ferret model has become indispensable for understanding the virulence and transmission of influenza viruses, partly because ferrets and humans share similar lung physiology," the paper says.

The CDC hasn't received any new reports of infection since December, which has scientists stumped.

"I wish we had a good answer for why it hasn't taken off in humans. We don't fully understand the factors involved," Tumpey says.

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The resulting flu from H3N2 viruses have generally been more severe than seasonal flu viruses, according to Tumpey. "Overall, the cases have been fairly mild, but there have been a few cases of hospitalization," he says.

From mid-August to late December 2011, the CDC received 12 reports of human infections from H3N2. The CDC has not reported any additional cases in 2012, but last week the organization warned that the 2012 season is the "latest flu season in nearly three decades" and that America will likely see more infections in the coming weeks.

"We've been lucky nothing has occurred so far in 2012," he says. "This study underscores the need for continued public health surveillance."