After five days of investigation, the world’s top flu experts say the H7N9 strain of bird flu that has emerged in China this spring is “definitely one of the most lethal influenza viruses that we've seen so far.”

WHO officials told reporters Wednesday that the H7N9 strain appears to be able to infect humans more easily than H5N1, the bird flu strain that started killing people a decade ago. And they’re concerned it could prove to be even more lethal.

“When we look at influenza viruses, this is an unusually dangerous virus,” Dr. Keiji Fukuda, the WHO’s top influenza expert, told a news briefing Wednesday.

So far, it doesn’t appear that H7N9 can spread easily from human to human. But Fukuda said that humans seem to catch H7N9 from birds more easily than the H5N1 strain.

H5N1 has killed 360 people worldwide since it was discovered in 2003 –- about 60 per cent of those it’s infected. H7N9 has been diagnosed in more than 100 people in China -- and one man from Taiwan who travelled to mainland China -- since it was discovered in March. It has killed at least 22 people.

Although H7N9 appears to have a lower fatality rate than H5N1, Fukuda suggested the picture of this virus is just emerging. He said it’s possible that authorities are hearing only about the most serious infections and there might be a large number of infections that are going unreported.

Wednesday's briefing came at the end of a five-day investigation by an international team of scientists led by the WHO, who worked with Chinese authorities in Beijing and Shanghai to try to better understand the new strain.

Fukuda says despite their work, it’s still not clear whether H7N9 might one day more easily spread from person to person.

"We have seen no sustained person-to-person transmission in any of these areas,” he said, later adding that the situation “remains complex and difficult and evolving.”

Fukuda said the investigation team still isn’t sure how people are getting infected with H7N9.

With H5N1, it became clear quickly that humans were most likely to become infected after close contact with infected poultry. But this new strain appears to be able to infect birds without causing any symptoms and that’s making it hard for experts to monitor its spread and understand how it infects humans.

So far, the evidence suggests that most human infections are occurring at live poultry markets, particularly through ducks and chickens.

This led investigators to report one important clue that might help contain the spread of the virus.

On April 6, Shanghai officials closed all poultry markets in the city and it appears the move may be working.

Anne Kelso, the Melbourne-based director of the WHO Collaborating Centre for Reference and Research on Influenza, said there had been a "dramatic slowdown” of new cases in Shanghai (which has recorded most of the deaths), since its live poultry markets were closed.

“It’s been very encouraging to see that almost immediately there was a decline in the detection of new cases,” Kelo said.

Nevertheless, the WHO is warning China to brace for continued infections.

That comes as Taiwanese health authorities confirmed their first human case of H7N9 – a case they said that appears to have been imported from China. The country’s Centres for Disease Control said a 53-year-old man became sick with H7N9 after returning from a visit to the eastern Chinese province of Jiangsu on April 9. It offered no details on his condition.

Meanwhile scientists in Canada are working to prepare in the event that the virus should somehow spread to this country.

Dr. Neil Rau told CTV News that the confirmed case in Taiwan has put several countries, including Canada, on alert for travellers who have recently been to China.

Dr. Allison McGreer, of Toronto’s Mount Sinai Hospital, said that unlike the 2003 SARS pandemic – where officials were dealing with a then-unknown disease-- if the H7N9 bird flu becomes the next pandemic, public health officials will be better prepared.

“If this is the next pandemic, it’s an influenza pandemic and it’s going to have the characteristics of an influenza pandemic,” she said.

Dr. Frank Plummer of the National Microbiology Laboratory in Winnipeg said scientists there are working with samples of the H7N9 virus to develop faster tests and treatments.

He said scientists there are starting to work on vaccine development, looking at anti-viral agents and producing antibodies that might be able to neutralize the virus.

“It’s all in the interest of being prepared in Canada in case this thing arrives here,” he said.

With a report from CTV News’ Medical Correspondent Avis Favaro and producer Elizabeth St. Philip