H7N9 flu still poses a pandemic threat

'More likely than other avian viruses to adapt.'

Even as transmission of H7N9 avian influenza subsided in China this spring, the Centers for Disease Control and Prevention remains at heightened concern over the potential for a new pandemic. Development of an H7N9 vaccine for clinical trials and possible stockpiling is already underway.

While acknowledging that pandemic prediction is extremely difficult, some of the nation's top virologists recently acknowledged that "H7N9 might arguably be more likely than other avian viruses to become human adapted."1

If so, the next global flu pandemic may be emerging in China, but they also added this hopeful counterpoint: "In 94 years of virologic surveillance, we have never seen a poultry-adapted influenza virus [i.e., H7N9] cause widespread human transmission."

As of June 20, CDC had tested samples from 67 suspect H7N9 cases in the United States; all were negative. CDC continued to urge health care providers to send respiratory specimens from suspect cases to the local or state health department – if there is severe acute respiratory infection that requires hospitalization and recent travel to China or contact with confirmed cases.

The threat of H7N9 emerged rather suddenly this spring when China notified the World Health Organization of three cases of human infection with avian influenza H7N9. By late June, there had been 132 lab-confirmed cases in China and 37 deaths, or a fatality rate of about 30%.In addition, concern is growing about H7N9 strains that are resistant to flu antivirals, meaning treatment options are few in the absence of vaccine. Severe illness occurred most often in older men with underlying health conditions. In fact, almost three out of four (72%) of the H7N9 patients were men. More than half of the patients were 60 or older. Researchers do not know why the disease has been found most frequently in older men, says Daniel Jernigan, MD, MPH, deputy director of CDC's influenza division.

"There's something different about this virus, and that's one reason that many people are very concerned about it," Jernigan told an advisory panel of vaccine experts, the Advisory Committee on Immunization Practices (ACIP).

Most cases involved exposure to live poultry. There have been five lab-confirmed clusters, but "it's difficult to know if there's human-to-human transmission," Jernigan said.

Existing immunity? None

Genetic sequencing of the virus showed that it has become more capable of binding to mammalian respiratory cells and replicating in those cells, he said. If H7N9 reemerges this fall, it could be problematic. "There does not appear to be any existing immunity in the U.S. population," said Jernigan.

Jernigan noted that the 132 confirmed cases occurred in just three months of the initial detection of H7N9 in China. (One of those cases was in Taiwan.) There have been just four small clusters that suggest human-to-human transmission. By contrast, H5N1 avian influenza emerged slowly, with just 18 cases in 1997 and a reemergence of human transmission in 2003.

Poultry infected with H5N1 become sick and often die, so ill flocks become a marker for transmission of disease. But poultry do not show signs of illness with H7N9. "We really can't monitor it very well in animals because they don't manifest disease," says Jernigan. And farmers have little incentive to test their flocks.

"Numerous factors cause [H7N9] to be concerning," he says.

Because so far most cases of H7N9 have involved severe disease, the CDC is targeting its surveillance on hospitalized patients with recent travel to China or exposure history.

Health care providers should test "only patients with an appropriate exposure history and severe respiratory illness requiring hospitalization," CDC said in a health advisory. Only confirmed and probable cases of human infection with H7N9 should be reported to CDC, the agency said.

Patients should be tested if they meet the criteria for both clinical illness and exposure, CDC said. Clinical illness is defined as patients with new-onset severe acute respiratory infection that requires hospitalization for which no alternative infectious etiology has been identified. Patients meet exposure criteria if they have traveled to an area where there have been human cases of H7N9 or where the avian influenza is known to be circulating in animals, or if they have had close contact with confirmed cases of H7N9.

In suspected cases, respiratory specimens should be sent to the local or state health department for testing, CDC said.

The infection control guidance for caring for patients with H7N9 including wearing an N95 (or more protective) respirator, eye protection, gown and gloves when entering a patient room. Updated information about H7N9 is available at www.cdc.gov/flu/avianflu/h7n9-virus.htm.


1. Morens DM, Taubenberger JK, Fauci AS. H7N9 avian influenza A virus and the perpetual challenge of potential human pandemicity. mBio 2013 Jul 9;4(4): Available at: http://mbio.asm.org/content/4/4/e00445-13