By September 19, an estimated 25 percent of the 352 death cases reported in the region had no prior medical problems, the WHO announced at its Western Pacific annual conference in Hong Kong.
"That worries us very much. We are looking into it," Takeshi Kasai, the organisation's regional adviser on communicable disease surveillance and response, told the media on the sidelines of the conference.
In other regions the proportion of swine flu fatalities with no underlying medical condition ranged from 20 to 50 percent, he said.
Kasai said one of the more plausible hypotheses virologists had come up with was that the virus replicated more rapidly in those patients.
He said that the WHO was also concerned that young adults were dying of swine flu, while small children and elderly people tend to be the main groups that succumb to seasonal influenza.
The West Pacific region covers 37 countries and extends from China and Mongolia to Australia, New Zealand and French Polynesia.