Geneva: The World Health Organization (WHO) is urging the planet to brace for a second wave of the swine flu pandemic as the heavily populated northern hemisphere edges towards the cooler season when flu thrives.

More trouble: People queued up to undergo a test at a special screening centre for A/H1N1 virus at RML hospital in New Delhi. Ramesh Pathania / Mint

WHO director general Margaret Chan warned on Friday that there had been second and third waves in previous pandemics.

“We cannot say for certain whether the worst is over or the worst is yet to come," Chan said in a videotaped address to a symposium on flu in the Asia-Pacific region.

“We need to be prepared for whatever surprises this capricious new virus delivers next," she added.

Some 1,799 people have died since the A/H1N1 was uncovered in Mexico and the US nearly six months ago, according to the UN health agency.

By comparison, an estimated 250,000-500,000 people die around the world every year from seasonal flu, and overall the symptoms of the new pandemic virus have proved to be mild in the great majority of known cases. However, it has spread swiftly into 177 countries, proving to be more infectious than seasonal flu.

Through a full season in the southern hemisphere, the pandemic strain gradually became dominant.

WHO monitoring showed that it was now on the decline there, except in South Africa, and in some later affected areas of Argentina, Australia and Chile.

It has cut its estimate of maximum annual production capacity of 4.9 billion vaccines, currently focusing on about half or even one quarter of that amount.