(AP) — The World Health Organization declared the 2009 H1N1 pandemic officially over Tuesday, months after many national authorities started canceling vaccine orders and shutting down hot lines as the disease ebbed from the headlines.

WHO Director-General Margaret Chan said the organization's emergency committee of top flu experts advised her that the pandemic had "largely run its course" and the world is no longer in phase six — the highest influenza alert level.

"I fully agree with the committee's advice," Chan told reporters in a telephone briefing from her native Hong Kong.

The virus has now entered the "post-pandemic" phase, meaning disease activity around the world has returned to levels usually seen for seasonal influenza, she said.

But Chan cautioned against complacency, saying that even though hospitalizations and deaths have dropped sharply, countries should still keep a watchful eye for unusual patterns of infection and mutations that might render existing vaccines and antiviral drugs ineffective.

"It is likely that the virus will continue to cause serious disease in younger age groups," she said, urging high-risk groups such as pregnant women to continue seeking vaccination.

Unusually, H1N1 influenza hits young adults harder than the over-65s, who are believed to have some immunity to the A (H1N1) strain.

At least 18,449 people have died worldwide since the outbreak began in April 2009. WHO said last week that the true figure is likely to be higher, but the organization's flu chief, Keiji Fukuda, said a final number won't be known for some months.

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Source: Associated Press