Centers for Disease Control and Prevention officials are developing a vaccine that would target a newly evolving version of the virus.

China is experiencing its largest outbreak of the H7N9 bird flu strain, with at least 460 infections reported since October. About a third of people diagnosed with H7N9 have died of their infections, according to the World Health Organization. Human infections with this type of bird flu were first reported in China in March 2013, and since then, there have been yearly epidemics of human infections.

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But this winter, the number of cases is greater than any of the previous four seasons. This year's infections account for more than a third of the 1,258 H7N9 cases that have been reported since 2013. Most human infections involve exposure to live poultry or contaminated environments, especially markets where live birds have been sold. During the previous four waves of H7N9, 88 percent of patients developed pneumonia, 68 percent were admitted to an intensive care unit and 41 percent died, according to a report from the CDC.

“This is the virus we were concerned about in 2013, and now we're seeing these increasing number of cases,” said Daniel Jernigan, who heads the CDC's influenza division, in an interview this week. “This year it came back much stronger, so the numbers of cases we're seeing has already surpassed all the other waves, and the season isn't even over yet.”

In addition, the virus has become more deadly to poultry, which might lead to more severe infections in humans, he said. For all those reasons, officials are watching developments closely.

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“This is a virus you don't want to take your eyes off,” he said.

Among a dozen animal and bird viruses that are not yet circulating widely in people, the H7N9 virus has the greatest potential to cause a pandemic, according to the CDC. That assessment is based on the virus's ability to spread easily and efficiently to people from animals and its ability to cause serious disease.

On Wednesday, WHO officials in Geneva said the risk of sustained human-to-human transmission of H7N9 remains low. The characteristics of the infections and case fatality rate remain similar to previous waves, officials said.

But “constant change is the nature of all influenza viruses,” Wenqing Zhang, who heads WHO's global influenza program, told reporters during a media briefing Wednesday. “This makes influenza a persistent and significant threat to public health.”

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One change already underway is that the virus has split into two distinct genetic lineages, with a new branch of the virus family now emerging in the current epidemic, officials said.

That has rendered the H7N9 vaccine stockpiled by the United States less effective against the newly emerging branch, officials said. The CDC is developing an influenza seed virus that can be used by vaccine manufacturers to produce another H7N9 vaccine to match a newly emerging H7N9 strain.

It will take several months to produce and test a new vaccine, a process that will get underway in June and July after vaccine manufacturers complete their work on making seasonal flu vaccine.

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Vaccines to protect first responders against the highest-risk bird flu viruses are part of the pandemic flu stockpile maintained by the Biomedical Advanced Research and Development Authority, the agency within the Department of Health and Human Services responsible for providing medical countermeasures to human-made and natural threats, including pandemic influenza and emerging infectious diseases.

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Rick Bright, director of BARDA, said manufacturers will be producing enough vaccine to provide 40 million doses, enough to vaccinate 20 million people, he said.