Speaking to Al Jazeera, Dr Laurence Gerlis, an expert on infectious diseases and anti-viral drugs, said: "I think at one level it is worrying, because there were many of us that thought that after a few weeks this would all of disappeared.
"The worrying thing for Africa, the Middle East and Europe is that as we go into autumn and the winter we are going to see something quite severe."
Announcing the decision, Margaret Chan, the WHO's director-general, said: "This is a very important and challenging day for all of us.
"It is important because we will be raising our pandemic alert level five to level six. At this time, the global assessment is that we are seeing a moderate pandemic.
"We are all in this together and we will all get through this together."
Chan also said countries already hit by H1N1 should prepare for a "second wave" of cases.
Michael Osterholm, a flu expert at the University of Minnesota who has advised the US government on pandemic preparations, said: "This is WHO finally catching up with the facts.''
The WHO pandemic alert had been at phase five since May 1, meaning it thought a global outbreak was imminent.
Thursday's announcement could put pressure on countries to activate their own pandemic preparedness plans if they had not already done.
This could include devoting more money to health services or imposing measures such as quarantines, school closures, travel bans and trade restrictions.