The deadly new bird flu may pose a bigger threat to humans than the H5N1 bird virus that has killed hundreds of people worldwide, a University of Hong Kong microbiologist warned yesterday.
Ho Pak-leung became the first expert to publicly express fears it could become a pandemic.
The H7N9 strain emerged in humans only last month and is so far contained to the mainland.
But it has already infected more people than the H5N1 virus has infected in the past 10 years.
Ho said the new virus showed a higher ability to be transmitted rapidly from birds to humans and to spread geographically.
And because infected birds appeared healthy, it was also harder to detect.
Ho said: "The pathology pattern of H7N9 so far is very special and quite different from H5N1.
"But the pattern points to one alarming conclusion - it may be a bigger health threat than H5N1."
He said news that a four-year-old Beijing boy was found to have the virus despite not showing any flu symptoms was a "warning sign" for a pandemic.
"It is possible for the virus to grow more adaptable to the human body … and eventually becoming transmittable among humans," he said.
The number of confirmed H7N9 cases has reached 63 - and 14 have died, according to the national health commission.
A boy who is now under medical observation in hospital was screened for the virus after coming into close contact with the capital's first H7N9 case, a seven-year-old girl. So far, H7N9 has emerged in Shanghai and the provinces of Zhejiang , Jiangsu , Henan and Anhui , as well as in Beijing.
"The previous H5N1 pandemic never affected so many provinces at the same time," Ho said.
H5N1 has infected 45 people on the mainland since 2003, killing 30.
Hong Kong was hit by an early outbreak in 1997, with six deaths, before the virus re-emerged in 2003 to spread throughout the world, claiming some 332 lives by late 2011.
Yuen Kwok-yuen, head of microbiology at HKU, said if infected people do not develop symptoms, the disease will get harder to control.
The husband of one of the H7N9 victims was confirmed to be infected with the virus 10 days after her death, sparking fears the virus could be passed between humans.
Respiratory diseases expert Dr Zhong Nanshan said it was too early to rule out the possibility of human-to-human transmission.
Zhong said: "We can only say that based on the evidence so far, no human-to-human transmission has been detected. But that does not mean it is not possible."
Click on each balloon for more information on individual patients infected with the avian flu virus: blue, patients infected with the H7N9 virus under treatment; red, those infected with the H7N9 who have died; and pink, those with H1N1 avian flu virus.