This winter, a resurgent wave of activity has produced more than a third of all infections ever recorded since the first hospitalised human case in February 2013 - 460 of 1,258 cases.
Changes in the virus are also worrying, said Dr Daniel Jernigan, director of the United States Centres for Disease Control and Prevention's (CDC) influenza department. A new strain of H7N9 bird flu virus has been confirmed earlier this month in two human cases in Guangdong province.
Resistance to drug raises concernIt could become drug-resistant to oseltamivir phosphate, a commonly used drug in the prevention and treatment of flu as seen in both the patients. However, oseltamivir phosphate has been effective for most human H7N9 cases, said Zhong Nanshan, a member of the Chinese Academy of Engineering.
"This shows most H7N9 viruses have not mutated to the new strain," he said.
He Jianfeng, chief expert in infectious disease at the Guangdong Provincial CDC said that although the new H7N9 strain demonstrated resistance to the drug, the strain might still have been affected by the drug.
"Both patients have used oseltamivir phosphate before, so it is not known whether the drug resistance is caused by previous use of the drug or by a mutation of the virus," he said. "But the possible drug-resistant nature of the new strain deserves more attention."
H7N9 might be the next pandemic threatThe new strain is also reportedly more dangerous to poultry, but still poses no new threat to humans and any increased risk, according to a statement by the Chinese CDC on 19 February. However unlike other H7N9 viruses, the new strain can cause diseases in poultry, which should be studied further.
"It certainly introduces uncertainty into the mix," Jernigan said, as what was once thought to be under control, has extrapolated quickly.
Until now, all of H7N9 infections have been contracted in China, but a few cases have involved tourists from elsewhere who were infected there. Approximately a third of people who have been diagnosed with H7N9 have had fatal infections, although experts note that milder cases could be occurring, which would lower the case fatality rate.
"The situation is not particularly reassuring at the moment in the field," said Professor Malik Peiris, head of the microbiology department at the University of Hong Kong. Being a veteran bird flu researcher, he called H7N9 "the most significant pandemic threat currently."
A US CDC risk assessment provided supporting evidence, placing H7N9 at the top of the list of pandemic threats from among a dozen bird and animal flu viruses.
Previous stock of vaccines may be useless in the futureWith the rapid rate of evolution of the virus, it raises concern that it will undermine the usefulness of a 12 million-dose emergency stockpile of H7N9 vaccine made for the US several years ago as it will mean that the vaccines are now less effective at targeting the strains of the virus that are circulating.
The H7N9 vaccine seed strain has been suggested to be changed due to this evolution by influenza experts who advise the World Health Organisation (WHO).
While the vaccines in the emergency stockpile could still offer some protection, "we think that there could be a better vaccine match," acknowledge Todd Davis, principal investigator on the CDC team that studies flu viruses that infect other mammals and birds.
H7N9 might spread to VietnamThe fear that H7N9 may not be contained to China itself anymore has increased as there have been rumoured reports that the virus has been found in provinces bordering Vietnam.
"I think now Vietnam is under very severe risk," said Peiris.
Scientists at the US CDC also called for a green light to share viruses from the Chinese government, to ensure that the flu drugs are still effective. But a disease diplomacy problem is posing a hurdle.
Meanwhile, the US has been developing their own samples through synthetic biology, where genetic sequences are used to develop sample viruses. However actual viral samples would be useful as synthetic biology still takes time. MIMS
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