Authorities are monitoring closely whether the virus was morphing into more virulent forms that would make it deadlier, the organization's Director-General Margaret Chan said.
"We are not seeing that situation right now," Chan told reporters as the WHO convened a conference in Hong Kong.
She said they are "monitoring the virus and seeing whether it has developed resistance to the current medicines that are effective." Specifically, she said they are seeing some cases of resistance to Tamiflu.
Tamiflu is one of two medicines that is used against swine flu, and health officials have been closely watching for signs that the virus is mutating, making the drugs ineffective.
Earlier this month, the maker of Tamiflu said it had reports of 13 cases of resistance worldwide that it called a very low rate.
The WHO says the swine flu virus - also known as H1N1 - has killed almost 3,486 people worldwide as of Sept. 13. South America and North America account for the majority of deaths.
For now, the infection is generally mild and most people recover without treatment. But should it become deadlier, developing nations could be especially vulnerable because those populations lack adequate health care and are already fighting a myriad of diseases including AIDS, malaria and tuberculosis.
Chan said manufacturers were on track to develop billions of new doses of the vaccine over the next year.