Risk of Guillain-Barré syndrome following H1N1 influenza vaccination in Quebec.
BACKGROUND
In fall 2009 in Quebec, Canada, an immunization campaign was launched against the 2009 influenza A(H1N1) pandemic strain, mostly using an AS03 adjuvant vaccine. By the end of the year, 57% of the 7.8 million residents had been vaccinated.


OBJECTIVE
To assess the risk of Guillain-Barré syndrome (GBS) following pandemic influenza vaccine administration.


METHODS
Population-based cohort study with follow-up over the 6-month period October 2009 through March 2010. The investigation was ordered by the chief medical officer of health in accordance with the Quebec Public Health Act.


METHODS
All acute care hospitals and neurology clinics in Quebec.


METHODS
Suspected and confirmed GBS cases reported by physicians, mostly neurologists, during active surveillance or identified in the provincial hospital summary discharge database. Medical records were reviewed and cases classified according to Brighton Collaboration definitions (categorized as level 1, 2, or 3, corresponding to criteria of decreasing certainty in diagnosis). Immunization status was verified and denominators were estimated from the provincial immunization registry (4.4 million vaccinated) and census data (total target population aged ≥6 months, 7.8 million), with a total of 3,623,046 person-years of observation.


METHODS
Relative and attributable risks were calculated using a Poisson model and the self-controlled case-series method.


RESULTS
Over a 6-month period, 83 confirmed GBS cases were identified, including 71 Brighton level 1 through 3 cases. Twenty-five confirmed cases had been vaccinated against 2009 influenza A(H1N1) 8 or fewer weeks before disease onset, with most (19/25) vaccinated 4 or fewer weeks before onset. In the Poisson model, the age- and sex-adjusted relative risk was 1.80 (95% CI, 1.12-2.87) for all confirmed cases during the 8-week postvaccination period and was 2.75 (95% CI, 1.63-4.62) during the 4-week postvaccination period. Using the self-controlled case-series method, relative risk estimates during the 4-week postvaccination period were 3.02 (95% CI, 1.64-5.56) for all confirmed cases (n = 42) and 2.33 (95% CI, 1.19-4.57) for Brighton level 1 through 3 cases (n = 36). The number of GBS cases attributable to vaccination was approximately 2 per 1 million doses. There was no indication of an excess risk in persons younger than 50 years.


CONCLUSIONS
In Quebec, the 2009 influenza A(H1N1) vaccine was associated with a small but significant risk of GBS. It is likely that the benefits of immunization outweigh the risks.
Department of Social and Preventive Medicine, Laval University, Quebec City, Quebec, Canada. Philippe.De.Wals@ssss.gouv.qc.ca
Philippe
P
De Wals
Geneviève
G
Deceuninck
Eveline
E
Toth
Nicole
N
Boulianne
Denis
D
Brunet
Renée-Myriam
RM
Boucher
Monique
M
Landry
Gaston
G
De Serres
D000276:Adjuvants, Immunologic; D000293:Adolescent; D000328:Adult; D000368:Aged; D002648:Child; D002675:Child, Preschool; D005260:Female; D020275:Guillain-Barre Syndrome / Q000453:epidemiology*; D006801:Humans; D017589:Immunization Programs; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype; D007252:Influenza Vaccines / Q000009:adverse effects*; D007251:Influenza, Human / Q000517:prevention & control*; D008297:Male; D008875:Middle Aged; D011159:Population Surveillance; D011792:Quebec / Q000453:epidemiology; D012306:Risk; D013997:Time Factors; D014611:Vaccination / Q000706:statistics & numerical data; D055815:Young Adult
D000276:Adjuvants, Immunologic; D007252:Influenza Vaccines