Reviewed by Dori F. Zaleznik, MD; Associate Clinical Professor of Medicine, Harvard Medical School, Boston.
By Michael Smith
Cases of childhood narcolepsy spiked in Finland in 2010, and researchers there are suggesting the adjuvanted vaccine against the H1N1 pandemic flu might have been a trigger.
Two related studies, appearing online in PLoS ONE, found that the incidence of narcolepsy rose markedly in children and adolescents, while remaining unchanged in those 20 and older.
Most of the cases in children occurred after vaccination with the ASO3-adjuvanted flu vaccine Pandemrix, which was the only vaccine used in Finland during the pandemic.
The two studies, with overlapping research teams, used hospital discharge data and vaccination records to identify cases and estimate incidence.
One study, led by Hanna Nohynek, MD, PhD, of the Finnish National Institute for Health and Welfare, focused on children and adolescents who were 4 to 19 in 2010.
The second study, led by Markku Partinen, MD, PhD, of the Helsinki Sleep Clinic, looked at children under 17, which is the cut-off for pediatric cases in Finland.
Narcolepsy is a chronic sleep disorder that causes excessive daytime sleepiness and cataplexy. It has a strong genetic predisposition, with specific immune system gene variations linked to onset.
In the 915,854 children and adolescents in the Nohynek study, 75% had been vaccinated, the researchers found, and there were 67 confirmed cases of narcolepsy.
Analysis showed an incidence of 9.0 per 100,000 person years among those had been vaccinated, compared with 0.7 per 100,000 person years in those who did not get the vaccine.
Those figures yielded an incidence rate ratio of 12.7, with a 95% confidence interval from 6.1 to 30.8, Nohynek and colleagues reported.
In the other study, Partinen and colleagues found that 335 cases of narcolepsy were diagnosed in Finland during the 7 years before the pandemic. That yielded an overall annual incidence of 0.79 per 100,000 inhabitants and 0.31 per 100,000 among those under 17, they reported.
But during 2010, they found, 54 children under 17 were diagnosed with narcolepsy, yielding a 17-fold increase in the incidence rate — to 5.3 per 100,000.
Fifty of the 54 had been vaccinated a median of 42 days before onset.
On the other hand, among those 20 and older, the 2010 incidence rate was 0.87 per 100,000, not different from the rate seen from 2002 through 2009.
Physicians performed genetic testing on 34 of the 54 children who were diagnosed with narcolepsy and found they were all positive for the narcolepsy risk allele DQB1*0602/DRB1*15.
The clinical picture, Partinen and colleagues reported, was similar for the most part to previously described childhood narcolepsy.
The 50 children with the risk allele all had excessive daytime sleepiness with multiple abnormal sleep latency tests. Their symptoms started abruptly and 47 had cataplexy, which started at the same time or soon after the onset of excessive daytime sleepiness.
Many of the children also had psychiatric symptoms, such as challenging and aggressive behavior or self-mutilation, the researchers found.
Both groups argued that the vaccine — and especially the highly immunogenic adjuvant — might have been a trigger that caused genetically predisposed children to develop the illness. That suggestion is bolstered by reports of a similar spike in Sweden, where Pandemrix was also the only vaccine used and where the prevalence of the risk alleles is similar.