World Health Organisation chief Margaret Chan warned Tuesday against complacency on the swine flu pandemic, noting that influenza viruses are "highly unpredictable" and could still mutate.
"One thing we need to guard aginst is a sense of complacency. Viruses are highly unpredictable. We should not be taken by surprise. That's why the WHO will continue to track the evolution. We will watch this virus with eagle's eyes," said the WHO director-general.
"Nobody wants to see the H1N1 virus mutate to a more dangerous form. Let's keep our fingers crossed," she added.
Even if several countries in the northern hemisphere have passed the peak of a second wave of infections, Chan pointed out that there is still intense transmissions in other countries.
"It is too premature and too early for us to say that we have come to an end of the pandemic influenza worldwide," she stressed.
The A(H1N1) virus has killed at least 11,516 people since it was first detected in March.
The rapid spread of the bug led the WHO to declare a pandemic in June. However, with most patients showing mild symptoms, the UN agency has been criticised by some for having over-reacted.
Chan assessed that it was fortunate that the pandemic turned out to have a milder impact than expected.
"The fact that the long overdue influenza pandemic is so moderate in its impact is probably the best health news in a decade," she said.
"If we are confronted with a severe pandemic, it could have brought the momentum of health development to a grinding halt and reversed all the hard-earned gains that the world collectively have achieved in the last 10 years," added Chan.
The UN health chief said that while the world is better prepared now than five years ago in dealing with a pandemic, "and yet the world is not prepared."
"We characterised the H1N1 pandemic influenza as a moderate pandemic. It has caused stress in the emergency rooms and in the ICUs (intensive case units) in many countries, and there are still gaps in the health systems in many countries," she said.
"Even handling a pandemic caused by H1N1 exposed some holes in the health system, I just wish that the world does not have to deal with a pandemic ignited by a much more toxic and deadly virus - the H5N1 avian flu virus."
"No, the world is not ready for a pandemic to be caused by H5N1," she added.
The WHO chief had estimated that the avian flu virus kills some 60 percent of infected people.