Narcolepsy and influenza A(H1N1) pandemic 2009 vaccination in the United States

Abstract

Objective: To assess the occurrence of narcolepsy after influenza vaccines used in the United States that contained the influenza A(H1N1)pdm09 virus strain.

Methods: A population-based cohort study in the Vaccine Safety Datalink with an annual population of more than 8.5 million people. All persons younger than 30 years who received a 2009 pandemic or a 2010-2011 seasonal influenza vaccine were identified. Their medical visit history was searched for a first-ever occurrence of an ICD-9 narcolepsy diagnosis code through the end of 2011. Chart review was done to confirm the diagnosis and determine the date of symptom onset. Cases were patients who met the International Classification of Sleep Disorders, 2nd edition, narcolepsy diagnostic criteria. We compared the observed number of cases after vaccination to the number expected to occur by chance alone.

Results: The number vaccinated with 2009 pandemic vaccine was 650,995 and with 2010-2011 seasonal vaccine was 870,530. Among these patients, 70 had a first-ever narcolepsy diagnosis code after vaccination, of which 16 had a chart-confirmed incident diagnosis of narcolepsy. None had their symptom onset during the 180 days after receipt of a 2009 pandemic vaccine compared with 6.52 expected, and 2 had onset after a 2010-2011 seasonal vaccine compared with 8.83 expected.

Conclusions: Influenza vaccines containing the A(H1N1)pdm09 virus strain used in the United States were not associated with an increased risk of narcolepsy. Vaccination with the influenza A(H1N1)pdm09 vaccine viral antigens does not appear to be sufficient by itself to increase the incidence of narcolepsy in a population.

Figure 1

Population incidence rate of narcolepsy diagnosis (first-ever narcolepsy ICD-9 diagnosis code in a patient’s record) for three age groups by quarter year, 2006–2011

Figure 2

Number of chart-confirmed incident narcolepsy cases by date of diagnosis (A) and date of symptom onset (B) among patients that received either MIV or MLAIV during the 2009 pandemic. Includes chart-confirmed incident cases of narcolepsy with cataplexy and narcolepsy without cataplexy diagnosed following receipt of a 2009 pandemic vaccine (n=11). The date of symptom onset was prior to the year 2009 for five of the cases and was not documented for one case - these six cases are not shown in the lower graph (B).