Health officials are scrambling to investigate a deadly new bird flu strain in China that has left two men dead and a woman fighting for her life, but so far, more questions than answers have emerged.

Is this the front edge of a major pandemic or just another blip on the world’s increasingly powerful radar for newly emerging viruses?

For now, flu experts say the only clear answer is this: it is too soon to tell.

“Is this the harbinger of things to come or is this just another sentence in an ongoing set of chapters about flu viruses?” said Dr. Michael Osterholm, an infectious disease expert with the University of Minnesota. “It’s too early to make any statement about global risk here. We’ve seen H7 viruses come and go before and that very well could be the case here.”

People have no natural immunity for the flu strain, called H7N9, and there is no readily available vaccine. But health officials say the virus does not appear to be spreading easily between people and no links have been identified between the three cases. They are now monitoring 88 contacts of the three victims and, so far, none has been infected — a good sign that momentarily dampens fears of a widening outbreak.

Influenza strains are categorized by the protein spikes on the surface of the virus and scientists are aware of 17 subtypes of the hemagglutinin protein (the “H”) and ten of the neuraminidase protein (the “N”). The new strain is from the H7 family, which has popped up before in both animals and humans.

But in the past, H7 strains have mostly caused mild sickness in people; the one notable exception is from 2003, when a veterinarian developed respiratory illness and died after an H7N7 outbreak in the Netherlands. Generally, however, infected people come down with eye infections or mild flu symptoms, as was the case in 2004, when an H7N3 outbreak in British Columbia infected two people and prompted the culling of 19 million poultry.

On Sunday, however, Chinese health officials announced that two Shanghai men have died in the first known cases of human infection with H7N9: an 87-year-old and a 27-year-old. The older man had two sons who also contracted pneumonia, one of whom died, but neither has yet to test positive for H7N9, according to a World Health Organization spokesperson.

The third person, a 35-year-old woman, was apparently infected in the city of Chuzhou in Anhui province, hundreds of kilometres northwest of Shanghai. She remains in critical condition.

All three people fell ill between Feb. 19 and March 15, according to the World Health Organization.

“To me, I think the most interesting aspect of the cases that were described were the geographical separation of the cases,” said flu expert Dr. Richard Webby, a virologist with St. Jude Children’s Research Hospital in Memphis, Tenn. This has two possible implications, he suggested; the virus is “either widespread in some poultry population — or the other more sinister explanation, which is it’s spreading amongst people.”

Some have speculated that the new virus could be linked with the thousands of pig carcasses recently found in China’s Huangpu River, which flows through Shanghai.

But Dr. Malik Peiris, a flu expert and professor of microbiology at the University of Hong Kong, has seen the DNA sequence of the new H7N9 strain and said there are no pig virus genes, only avian virus genes. Chinese newspapers are also reporting that 34 pig carcasses from the river have now been tested and none has any trace of bird flu.

For Peiris, there are now two outstanding issues of particular urgency. The first is to determine whether the virus may already be circulating in people undetected, meaning the three cases are just the tip of the iceberg. The second is to locate the animal that is harbouring the virus.

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One worry is that H7N9 could become like H5N1, the most famous of the avian flu strains that first emerged in Hong Kong in 1997, killing six people. H5N1 has now spread to 15 countries, occasionally decimating bird flocks and causing more than 320 human deaths in the last decade.

With the new flu strain, there is now a small window of time to track down the animal source and stem the spread of the virus, Peiris said. He concedes it may already be too late but “we need to make the effort.”

“If you can detect that early, you can have a chance of stamping it out in the animal reservoir,” said Peiris. “Once you miss that chance, then of course it becomes impossible.”

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