A possible link between cases of the chronic sleep disorder narcolepsy and GlaxoSmithKline’s H1N1 pandemic vaccine, Pandemrix, has led to a call from the World Health Organization (WHO) for further investigation. Narcolepsy is a rare condition with no currently available cure.

Cases of narcolepsy with a potential connection to the vaccine first began hitting headlines in August 2010, prompting the European Medicines Agency (EMA) to launch a safety review. Six months on, however, it’s not entirely clear what’s going on. Is there a connection? Possibly, but only in certain countries and a certain age group! Although cases of narcolepsy have been reported in at least 12 countries, the majority of these come from Sweden, Finland and Iceland in children and adolescents aged 4–19 years.

“Even at this stage, it does not appear that narcolepsy following vaccination against pandemic influenza is a general worldwide phenomenon and this complicates interpretation of the findings in Finland,” explained a statement from the WHO.

The issue is also further confounded by the fact that an increased risk of narcolepsy has never been observed in association with the use of any vaccine — whether against influenza or other diseases — before.

Finland’s National Institute for Health and Welfare has been heavily involved in investigating the issue and established a National Narcolepsy Task Force last year. According to a report from the task force, 60 children and adolescents aged 4–19 years fell ill with narcolepsy in 2009–2010. Of these cases, almost 90% had received Pandemrix.

“Based on the preliminary analysis, the risk of falling ill with narcolepsy among those vaccinated in the 4–19 years age group was 9-fold in comparison to those unvaccinated in the same age group,” said a press statement from the National Institute for Health and Welfare.

Who’s to blame?

But given that no other vaccine has ever caused narcolepsy before, is Pandemrix really to blame?

A number of countries used similar pandemic vaccines in 2009, but the sharp increase in narcolepsy cases only seems to have occurred in Sweden, Finland and Iceland. Iceland has also reported greater than expected numbers of narcolepsy cases in unvaccinated children and teenagers in the country. GSK believes it’s too early to draw conclusions until the European investigation has been completed, but the company says it is working closely with the EMA to understand the situation.

Narcolepsy has also been shown to have a strong genetic linkage to a specific genotype, which all of the people diagnosed with narcolepsy during 2009–2010 in Finland possessed. The WHO explained that the National Institute “considers it most likely that the Pandemrix vaccine increased the risk of narcolepsy in a joint effect in those genetically exposed with some other, still unknown, genetic and/or environmental factor”.

For now, there’s no clear picture of exactly what is happening. According to GSK, more than 31 million doses of the vaccine have been administered worldwide in 47 countries, with only 162 cases of narcolepsy reported to GSK as of 31 January 2011.

The European Centre for Disease Prevention and Control, in collaboration with a number of EU member states, including Finland, is currently conducting epidemiological studies of narcolepsy and pandemic influenza vaccines. These studies will evaluate the contribution of pandemic vaccines and other risk factors to narcolepsy, and confirm whether an increase in narcolepsy has been seen in other countries. The outcomes of the study will be announced this summer.