The WHO also has no evidence of widespread resistance to antiviral medication, Fukuda said.
Other useful protections against H1N1 are vaccines, which the WHO recommends against pandemic infections, Fukuda said.
"We welcome the announcement, " government spokesman Yasuo Fukuda said at a press conference.
But Mr Fukuda said the WHO "certainly has no recommendations on whether women should try to have children now".
In Central America and the Southern Hemisphere, however, activity levels have dropped as those regions enter the summer season, Fukuda said.
But contrary to some reports, Fukuda said, the WHO has not seen big mutations in the virus since it first emerged.
"East Asia is one of the parts of the world where seasonal influenza viruses have remained in reasonably high circulation, " Fukuda said.
In Australia, for example, "aboriginal groups are disproportionately represented in people who end up in hospitals from diseases related to the pandemic, " Fukuda said.
Mr Fukuda said officials and drug manufacturers were investigating how to speed up the process of developing a vaccine against the H1N1 swine flu strain.
"This is definitely one of the most lethal influenza viruses we have seen so far, " WHO flu expert Dr Keiji Fukuda said at a news conference in Beijing.
The only side effects are swelling and pain at the injection site, but "these are occurring at rates that are expected and usually seen with seasonal influenza vaccine, " Fukuda said.
Disease activity has been difficult to predict, Fukuda said.
The move means the U.N. agency has determined that the virus is capable of significant human-to-human transmission -- a major step toward a pandemic, but not necessarily inevitable, Dr. Keiji Fukuda said.
"In this age of global travel, where people move around in airplanes so quickly, there is no region to which this virus could not spread, " said Fukuda, assistant director-general of the WHO.
Researchers are still trying to determine how easily the virus is transmitted person to person and it's too early to predict whether there will be a mild or serious pandemic, said Fukuda.
It was Mr Fukuda, it is said, who brokered the recent diplomatic reconciliation with China, for instance.
"This naturally-occurring cholesterol offers a very specific target for the design of safer drugs that could treat stomach ulcers and, long-term, prevent stomach cancer linked with H.pylori, " said Dr Minoru Fukuda, who led the study.
The move from level three to level four on the WHO's six-level threat scale means the world body has determined the virus is capable of significant human-to-human transmission -- a major step toward a flu pandemic, said Dr. Keiji Fukuda, the agency's assistant director-general.
"We continue to see human-to-human transmission, community transmission, primarily in North America, " said WHO Assistant Director-General Keiji Fukuda.
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