In a CDC report released Friday, U.S. officials said among all emerging influenza viruses, the H7N9 bird flu poses the greatest risk of a pandemic threat if it evolves into a virus that spreads easily via human-to-human transmission.
The report follows an announcement from the World Health Organization last week stating that while the H7H9's risk of sustained transmission among humans is low, a recent uptick in human cases in China this winter and changes in the way the virus presents itself in humans means that the situation needs constant monitoring. CDC officials currently are developing a vaccine designed to target the evolving strain of the virus, the Washington Post's "To Your Health" reports.
Record number of cases this winter
There have been 460 laboratory-confirmed human cases of H7N9 bird flu in China this winter, according to Wenqing Zhang, head of WHO's global influenza program. That's the highest number of cases reported in any flu season since the first report of a human case in 2013, according to the New York Times. Though Yuelong Shu, an influenza expert at China's Center for Disease Control and Prevention, said this winter's outbreak appears to have peaked, as new cases of H7N9 are on the decline.
STAT News reports that so far, all of the H7N9 infections have been contracted in China, including some cases involving tourists from other countries who were infected while visiting. About one-third of individuals who contract H7N9 have died from the infection, according to STAT News. However, experts also note that undetected mild cases are probably occurring as well. Nearly all cases in humans were transmitted from birds, the Times reports. Zhang said there have been a few cases of human-to-human transmission but that there is no evidence of "sustained human-to-human transmission."
The recent wave of H7N9 flu activity represents more than a third of cases recorded since the first human case was reported—and it also means that H7N9 has overtaken another bird flu, H5N1, that has sporadically infected humans for at least a decade longer than the H7N9 strain. According to STAT News, prior to the latest uptick, H7N9 "had seemed over the past couple of years to be diminishing as a threat."
While the surge in cases this winter has been concerning, WHO said there is no evidence of a change in the human characteristics of infection, such as age, exposure history, and case fatality rate.
Zhang said about 7 percent of human cases of H7N9 this year are showing resistance to neuraminidase inhibitors—the antiviral drugs included in flu treatment's such as Roche's Tamiflu. However, individuals who showed signs of resistance had already been treated with antivirals, suggesting that the virus might have developed a resistance in their bodies, as opposed to them getting infected by an already resistant strain.
Therefore, WHO recommended that treatment for infection remain the same. "There is no evidence to recommend changes in clinical management," Zhang said.
In addition, researchers said they found genetic changes in three recent human cases of H7N9 that could make the virus more pathogenic to birds. According to WHO, the potential for the virus to pose a severe health risk to birds was previously low, so the changes serve as a reminder that viruses are constantly evolving and must be monitored.
Citing those genetic changes, Daniel Jernigan, director of CDC's influenza branch, said, "We're concerned about the uncertainties here and the number of changes that are happening at this point." However, he said that the increased severity of the virus among birds could have a beneficial effect if it spurs poultry markets to more rigorously enforce containment efforts that have grown lax over the last few years.
CDC said it classified H7N9 as the greatest pandemic threat among emerging influenza viruses because of its potential to spread easily from animals to humans and its ability to cause serious disease, "To Your Health" reports.
According to STAT News, the way in which the virus has evolved means that an existing H7N9 vaccine—which the United States stockpiled several years ago—is now less effective. Todd Davis, principal investigator on the CDC team that studies flu viruses that infect other mammals and birds, said the existing vaccine would likely still confer some protection but that scientists "think that there could be a better vaccine match."
Jacqueline Katz, deputy director of CDC's flu division, said U.S. scientists have been developing an H7N9 vaccine seed strain—the virus used as a target for organizations developing vaccines—which they could use to make an emergency stockpile in case the virus became more lethal or more easily transmissible.
Rick Bright, director of HHS' Biomedical Advanced Research and Development Authority, said drugmakers intend to make about 40 million doses of the new vaccine, which could be used to vaccinate up to 20 million people. According to "To Your Health," it will take several months to develop and test a new vaccine (Sun, "To Your Health," Washington Post, 3/3; Branswell, STAT News, 2/28; McNeil, New York Times, 3/1; Kelland, Reuters, 3/1; Senthilingam, CNN, 1/2).
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