"We are in the earliest days of the pandemic," Dr. Margaret Chan said in announcing the pandemic alert level was raised from Phase 5 to Phase 6, the highest level. "The virus is spreading under a close and careful watch."
WHO statistics indicate 28,774 H1N1 cases and 144 deaths have been confirmed in 74 countries as of Thursday.
"The virus is contagious, spreading easily from one person to another, and from one country to another," Chan said.
No previous pandemic was detected so early or watched so closely at the very beginning, Chan said. Close monitoring, diligent investigating and good reporting provided "early snapshots" of the virus's spread and range of illness, she said.
"Globally, we have good reason to believe that this pandemic, at least in its early days, will be of moderate severity," Chan said. "On present evidence, the overwhelming majority of patients experience mild symptoms and make a rapid and full recovery, often in the absence of any form of medical treatment."
She noted, however, that the early picture could change quickly.
"The virus writes the rules and this one, like all influenza viruses, can change the rules, without rhyme or reason, at any time," she said.
WHO has been working with flu vaccine manufacturers to ensure the "largest possible supply of pandemic vaccine in the months to come," she said. "Pending the availability of vaccines, several non-pharmaceutical interventions can confer some protection."
WHO continued to recommend no restrictions on travel and no border closures.