Japan's health ministry has stepped up monitoring of the spread of the H7N9 flu virus after studies warned it could trigger a pandemic because Japanese people lack natural immunity.
Scientists at the University of Tokyo determined that of the blood samples taken from 500 Japanese test subjects, none contained H7N9 antibodies.
Yoshihiro Kawaoka, who led the research, said only a limited number of people were taken ill in the 2009 outbreak of the H1N1 virus because adults had a degree of natural immunity to the strain.
But he said: "Nobody is immune to H7N9, which means it could have serious ramifications in humans."
Another study by scientists at the University of Wisconsin has discovered that the new virus is more pathogenic in mice than the H1N1 strain and replicates efficiently in both the upper and lower respiratory tracts of non-human primates. In comparison, human flu viruses are known to affect only primates' upper airways.
The studies support the theory that the virus could evolve to efficient human-to-human transmission, according to the Atlanta-based Centres for Disease Control and Prevention
The virus is believed to have originated in mainland China and has spread to Taiwan.
Tomoya Saito, deputy director of the Health Bureau at the Ministry of Health, Labour and Welfare in Tokyo, told the Sunday Morning Post that a range of preventive measures have already been taken.
"We have around 20,000 visitors arriving in Japan every day from China, aboard 144 flights and into 20 airports around the country," he said.
"We think there is a very high risk of a case being imported into Japan."
Efforts to combat the spread of the disease to date include distributing leaflets at arrivals terminals, asking anyone with symptoms of the disease to seek medical attention and the establishment of a laboratory with specialised diagnosis facilities.
"We have designated H7N9 under the Infectious Diseases Control Act, which enables us to perform medical checks on anyone we suspect may have been infected at their point of entry into Japan," he added.
Saito played down suggestions that Japan alone faces the threat of a pandemic, pointing out that as the H7N9 strain is completely new, studies in other countries would be expected to return the same results.
"But we are very closely watching the epidemiological situation before we make a decision on raising the alert level," he said.
Determining that the virus can be transmitted through human-to-human contact would be sufficient to prompt that decision, he confirmed.
At least 43 people died in China when the infection first appeared earlier this year, although it began to taper off early last month.
View H7N9 map in a larger map
Click on each balloon for more information on individual patients infected: blue, patients infected with the H7N9 virus under treatment; red, those infected with H7N9 who have died; yellow, those who have fully recovered; and pink, those infected other types of the Influenza A virus, including H1N1.