Vaccination with the adjuvanted pandemic influenza A H1N1 vaccine is associated with a 14- to 16-fold increase in a child’s likelihood of developing narcolepsy, a British study indicates.
Approximately one case of narcolepsy occurred per 57,500 to 52,000 doses of AS03-containing influenza A H1N1 (Pandemrix) administered in England and Europe in 2009, Elizabeth Miller, MBBS, of the Health Protection Agency in London, and colleagues reported online in BMJ.
These findings are consistent with previous studies that found a 13- to 17-fold increased narcolepsy risk among children in Finland and Scotland who were given the same vaccine.
The H1N1 pandemic flu vaccine used in the United States did not contain AS03 adjuvant, and there have not been any associated reports of narcolepsy. There is no association between the AS03-adjuvanted influenza A H1N1 vaccine and narcolepsy in adults, or with the seasonal influenza vaccine at any age.
In order to confirm the findings from Finland and Scotland, Miller and colleagues performed retrospective case reviews to identify narcolepsy diagnoses in children aged 4 to 18 years at all sleep centers and pediatric neurology centers in England.
The researchers identified a total of 75 children whose medical records indicated they had received a narcolepsy diagnosis with onset after Jan. 1, 2008. Among these cases, 11 children had narcolepsy onset after received the pandemic vaccine, as follows:
- Six within 3 months after vaccination
- One within 3 to 6 months
- Four between 7 and 14 months
The odds of receiving a narcolepsy diagnosis by July 2011 increased 14.4 fold if participants were vaccinated with the pandemic vaccine any time before onset and increased 16.2 fold if vaccination occurred within 6 months prior to onset. Both of these findings were statistically significant.
However, the researchers cautioned that the association is speculative, that it does not establish causation, and that a mechanism of action has yet to be identified.
“Despite attempts to minimize ascertainment bias, the potential for overestimation of risk remains because of more rapid referral of vaccinated patients,” the researchers wrote. “Long term follow-up of the cohorts exposed to the vaccine is needed to properly evaluate the attributable risk.”
In the meantime European health officials have restricted use of the the AS03-adjuvanted H1N1 vaccine among patients aged younger than 20 years who do not have access to seasonal trivalent, and for those whom prevention of influenza A H1N1 2009 influenza is considered necessary.
The license for Pandemrix, however, remains valid in the European Union, so it can still be manufactured and sold in any European Union country. An FDA panel also recently endorsed a vaccine that contains the AS03 adjuvant to protect against H5N1 bird flu.