The swine flu virus has not mutated into a more deadly strain but there are early signs it is developing resistance to vaccine, the World Heath Organization's chief said Monday.
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.
Chan said manufacturers were on track to develop billions of new doses of the vaccine over the next year. The vaccine is highly effective against the swine flu virus, though there were a small number of instances - about 25 in the world - of a vaccine-resistant flu.