World Health Organization chief Margaret Chan on Friday urged governments to prepare for a likely second wave of swine flu cases, cautioning they will face tough decisions on how to dispense vaccines.
"We cannot say for certain whether the worst is over or the worst is yet to come," Chan said via videotaped address at the start of a three-day symposium on influenza in the Asia-Pacific region.
"We need to be prepared for whatever surprises this capricious new virus delivers next... constant random mutation is the survival mechanism of the microbial world.
"We also need to prepare for a second or even a third wave of spread as typically seen in past pandemics."
About 1,800 people have died since A(H1N1) influenza was first uncovered in April, according to the latest update from the WHO issued this week. The vast majority of those deaths have been recorded in the Americas.
The WHO declared a global pandemic in June, and the agency now says there are confirmed cases in more than 170 countries.
While the epidemic appears to be peaking in the southern hemisphere, pandemic preparations should be stepped up in the northern hemisphere as the seasonal flu season approaches, Chan said.
"Like all influenza viruses, H1N1 has the advantage of surprise on its side," she said.
"We have the advantage of science and rational investigation on our side, supported by ... data collection, analysis and communication that are unprecedented in their power."
Chan said the issue of vaccine supplies needed to be tackled "head on", as more than two dozen pharmaceutical companies around the world scramble to produce safe and effective vaccinations.
"We need to gather advice on priority groups for initial protection," Chan said.
"This is one of the most difficult decisions governments around the world will need to make, especially as we know that supplies will be extremely limited for some months to come."
The WHO said earlier this week that countries in the northern hemisphere alone had ordered more than one billion doses of swine flu vaccine, sparking warnings about shortages, given the intense demand and production delays.