By Michael Smith

But risk of sustained transmission among people remains low: WHO

The current wave of H7N9 avian flu in China is the largest yet and accounts for more than a third of the human cases recorded since the strain was identified in 2013, according to an official of the World Health Organization.

But most other characteristics of the current outbreak, now standing at 460 cases, are similar to earlier waves, including the median age of patients, their history of exposure to poultry, and the risk of dying from the disease, said Wenqing Zhang, MD, head of the agency's global influenza program.

Risk of sustained transmission between people remains low, she told reporters in a telephone briefing, but "constant change is the nature of all influenza viruses (making) influenza a persistent and significant threat to public health."

Avian H7N9 influenza was identified in humans in 2013 and experts were concerned at the time that it had two of the three characteristics that could lead to a pandemic -- it was able to infect people and because it was new few people had any immunity to it.

The missing link was the ability to spread efficiently among people and so far, Zhang said, the virus hasn't acquired that capacity.

The current outbreak in China appears to have peaked, commented Yuelong Shu, PhD, director of the WHO Collaborating Center on influenza at the China Center for Disease Control in Beijing.

But Zhang said it's "highly likely" that more sporadic cases will be reported, because the virus is still circulating in poultry in China and people shopping in live poultry markets are likely to be exposed, despite calls for them to be closed.

She added that genetic analysis of isolates from three patients suggests changes that might make the strain more pathogenic among birds, although that changes don't affect pathogenicity or transmissibility in humans.

Indeed, Shu told reporters that in the two such cases in mainland China, one patient has recovered and been released, while the other remains in hospital because of a chronic disease not related to the flu. (The third case was reported from Taiwan.)

The virus ordinarily does not cause disease among birds, making it difficult to track. In contrast, the H5N1 avian flu -- which first caused concern in 1997 -- is highly pathogenic to birds, causing widespread death in poultry flocks.

Other genetic changes seen in the current wave of human H7N9 flu include markers associated with resistance to some of the drugs used to treat influenza, Zhang said. The changes are in the neuraminidase protein, which might indicate resistance to oseltamivir (Tamiflu) and zanamivir (Relenza), which inhibit neuraminidase.

Shu added that the resistance markers for the most part have been seen in samples from patients after treatment, suggesting they are a result of therapy. Circulating viruses don't appear to be resistant, he said.