The WHO advisory panel statement on Tuesday came after a Finnish study last week suggested children given the GSK shot were nine times more likely to suffer from narcolepsy, which causes a person to fall asleep suddenly and unexpectedly.
WHO spokeswoman Alison Brunier told a briefing in Geneva that Pandemrix remains on the WHO's list of prequalified vaccines, and there would be no changes to WHO recommendations on any flu vaccines as a result of the Finnish study.
"Further investigation is warranted concerning narcolepsy and vaccination not only with Pandemrix, but also with other pandemic H1N1 vaccines," Brunier said.
She said countries should continue immunizing people at risk of severe disease using pandemic vaccines, including Pandemrix, if no seasonal vaccines are available."
A spokesman for the British drugmaker said it had been expecting the WHO statement and was itself looking into reports of links between narcolepsy and Pandemrix.
"GSK is reviewing the Finnish report and believes it would be premature to draw any conclusions," he said in a statement.
Pandemrix was designed to tackle the H1N1 pandemic flu which spread around the world in 2009 and 2010.
It is not widely used currently as the pandemic was declared over in August last year. But doctors in some countries, such as Britain, have been advised to use it if stocks of seasonal flu vaccines run low.
The United Nations health body said an increased risk of narcolepsy had not been seen in association with the use of any other vaccines, either against flu or other diseases. "Thus far the study groups have been concentrating on Pandemrix. But the conclusions of the (WHO panel) is that they should look at other pandemic vaccines as well," Brunier said. "So the studies will go beyond just Pandemrix.
Several drugmakers, including Novartis, Sanofi-Aventis, CSL and Baxter made vaccines against H1N1 flu during the pandemic.
The WHO said the Stockholm-based European Centre for Disease Prevention and Control (ECDC), which monitors disease in the European Union (EU), is currently conducting studies of narcolepsy and pandemic flu vaccines.
"The findings from these studies and others, including further investigations in Finland, may help clarify the determinants of any increased risk," it said.
Researchers at Finland's National Institute for Health and Welfare last week suggested it was "most likely" the increase they found in narcolepsy was a joint effect of Pandemrix and some other factor or factors.
According to GSK, more than 31 million doses of Pandemrix have been administered worldwide in 47 countries. The company said it had received reports of a total of 162 cases of narcolepsy as of Jan. 31, 2011, with 70 percent of these cases of narcolepsy originating from Finland and Sweden.
The European Medicines Agency, which regulates and reviews the safety of drugs in the EU, said in September it was reviewing Pandemrix after reports of a link between the vaccine and narcolepsy.