An H1N1 pandemic flu vaccine may be linked to a severe form of narcolepsy in children and adolescents in Sweden, finds a new country-wide study by the country’s Medical Products Agency.

The findings support a preliminary study of four regions of Sweden, released in March, as well as a January report issued by Finland’s National Institute for Health and Welfare supporting this connection.

However, none of these studies prove that the vaccine causes narcolepsy nor explains how the vaccine might possibly cause the condition. Moreover, other countries in which the vaccine was administered have not detected a similar connection.

The new report finds that Swedes under 19 who received GlaxoSmithKline’s Pandemrix vaccine were more than 6 times likelier to develop narcolepsy with cataplexy, or sudden loss of muscle tone, within three months of vaccination compared with those who did not get a flu jab.



Between 2009 and 2010, when fears of an influenza pandemic prompted widespread vaccination, the study determined that 81 Swedes under the age of 20 developed narcolepsy with cataplexy, which is the most severe and easily defined form of the condition.

Of those cases, 69 patients received the H1N1 vaccine before they developed narcolepsy. All told, vaccinated Swedes were 6.6 times likelier to develop narcolepsy with cataplexy than those who did not get the vaccine. The difference was even starker when the researchers examined cases that occurred within three months of vaccination: those that got Pandemrix proved about 10 times as likely to develop narcolepsy as unvaccinated Swedes.

The researchers say their study is particularly strong because it was based on medical records examined by three professionals to detect cases of narcolepsy with cataplexy as thoroughly as possible. After these cases were firmly established, irrespective of vaccination status, the researchers determined who got Pandemrix and who didn’t.

No epidemiological study, particularly a retrospective one such as this, is perfect. National rates of narcolepsy aren’t known in Sweden, so the researchers can’t be sure they caught every case. What’s more, widespread media coverage of the link could have biased reporting of narcolepsy cases by children, their parents and even their physicians, the researchers note. The study did not take into account potentially confounding factors. It’s also worth noting that narcolepsy is quite rare and that Pandemrix was associated with an increase in just 3.6 cases per 100,000 Swedes.

Results of a more rigorous case-controlled study into the Pandemrix-narcolepsy connection, sponsored by the European Centre for Disease Control, are expected soon.

For more on vaccine safety issues in general, check out Nature’s excellent recent feature on the topic.