Background: In Taiwan, H1N1 vaccination began on November 1, 2009 and coincided with peak H1N1 pandemic activity. Nationwide ecological and case-control studies have identified no substantial association between the use of H1N1 vaccines and narcolepsy; however, wild-type H1N1 virus infection might have triggered narcolepsy onset, or potentially confounded the findings.
Methods: Data collected in the nationwide case-control study was reanalyzed. Confirmed narcolepsy cases (Brighton levels 1–2 for ages 0 to 15 years and 1–4a for ages at least 16 years) with onset during November 1, 2009–September 30, 2010 were included and ascertained receipt of H1N1 vaccines. We compared incidence of narcolepsy between the H1N1 vaccinated and unvaccinated population and assessed daily cumulative risk throughout the study period, with adjustment for age. We applied population estimates (census data, 2009) and daily doses of H1N1 vaccines administered (Influenza Vaccine Information System) to calculate the number of persons and person-time for each group.
Results: There were 22 narcolepsy cases; 5 (23%) occurred after H1N1 vaccination. The vaccinated population had higher incidence (1.2 vs 1.0 per million person-years, p = 0.711) (incidence rate ratio 1.24, 95% confidence interval [CI] 0.40–3.83), and higher cumulative risk (1.1 vs 0.9 per million persons, p = 0.772) (risk ratio 1.16, 95% CI 0.43–3.14) of narcolepsy (Figure). These differences, however, were not significant.
Conclusion: We found comparable average and cumulative risk of narcolepsy between the H1N1 vaccinated and unvaccinated Taiwanese population during the 2009–2010 pandemic.
Figure. Daily cumulative risk of narcolepsy among the H1N1 vaccinated and unvaccinated groups, November 1, 2009–September 30, 2010
W. T. Huang,