Dr Mike Ryan, WHO director of epidemic and pandemic alert response, said in recent months there had been a decrease incases in areas like Canada and the US.
However, warning that this could mean the virus is going through a “transitional event”, the Irishexpert said there had been an “upswing” in cases in tropical regions which could see a second wave develop later this year.
“We’re starting to see upswings in cases in tropical countries,” confirmed Dr Ryan.
“That may represent a transitional event and we may be seeing the virus establishing itself in some of those more tropical zones.
“There is always a possibility with influenza viruses that they are likely to change. This virus hasremained remarkably stable, but we have to be prepared for that possibility,” he added.
Dr Ryan was speaking after reports that initialdata from the soon-to-be-produced swine flu vaccine had indicated positive preliminary results, with suggestions one dose could be enough to inoculate an individual.
However, while welcoming the news, the WHO official said it was too early to determine the effectiveness of the vaccine.
“We’re seeing someencouraging results from China on preliminary results from trials, and we’re waiting for the next one or two weeks for other results from trials in countries like the US, Japan and Britain.
“But it’s important to remember that this [the reports from China] are preliminary results and have not yet been presented to the regulatory authority in China.
“From what we have seen the blood response to the vaccine appears pretty good, but it remains to be seen what the regulatory body will make of that data,” he said.
With countries across the globe seeking access to the swine flu vaccine for their populations, Dr Ryan said it is inevitable supply will outstrip demand.
The Irish expert said the situation will only impact on countries such as Ireland – which has already ordered vaccine doses – in terms of a “rationing” of the initial vaccine.