Prevalence of seroprotection against the pandemic (H1N1) virus after the 2009 pandemic.
BACKGROUND
Before pandemic (H1N1) 2009, less than 10% of serum samples collected from all age groups in the Lower Mainland of British Columbia, Canada, showed seroprotection against the pandemic (H1N1) 2009 virus, except those from very elderly people. We reassessed this profile of seroprotection by age in the same region six months after the fall 2009 pandemic and vaccination campaign.


METHODS
We evaluated 100 anonymized serum samples per 10-year age group based on convenience sampling. We measured levels of antibody against the pandemic virus by hemagglutination inhibition and microneutralization assays. We assessed geometric mean titres and the proportion of people with seroprotective antibody levels (hemagglutination inhibition titre ≥ 40). We performed sensitivity analyses to evaluate titre thresholds of 80, 20 and 10.


RESULTS
Serum samples from 1127 people aged 9 months to 101 years were obtained. The overall age-standardized proportion of people with seroprotective antibody levels was 46%. A U-shaped age distribution was identified regardless of assay or titre threshold applied. Among those less than 20 years old and those 80 years and older, the prevalence of seroprotection was comparably high at about 70%. Seroprotection was 44% among those aged 20-49 and 30% among those 50-79 years. It was lowest among people aged 70-79 years (21%) and highest among those 90 years and older (88%).


CONCLUSIONS
We measured much higher levels of seroprotection after the 2009 pandemic compared than before the pandemic, with a U-shaped age distribution now evident. These findings, particularly the low levels of seroprotection among people aged 50-79 years, should be confirmed in other settings and closer to the influenza season.
BC Centre for Disease Control, Vancouver, BC. danuta.skowronski@bccdc.ca
Danuta M
DM
Skowronski
Travis S
TS
Hottes
Naveed Z
NZ
Janjua
Dale
D
Purych
Suzana
S
Sabaiduc
Tracy
T
Chan
Gaston
G
De Serres
Jennifer
J
Gardy
Janet E
JE
McElhaney
David M
DM
Patrick
Martin
M
Petric
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral / Q000097:blood / Q000276:immunology*; D001955:British Columbia / Q000453:epidemiology; D002648:Child; D002675:Child, Preschool; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D007223:Infant; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007252:Influenza Vaccines / Q000276:immunology / Q000627:therapeutic use; D007251:Influenza, Human / Q000276:immunology* / Q000517:prevention & control; D008297:Male; D008875:Middle Aged; D058873:Pandemics / Q000517:prevention & control / Q000706:statistics & numerical data*; D014611:Vaccination / Q000706:statistics & numerical data; D055815:Young Adult
D000914:Antibodies, Viral; D007252:Influenza Vaccines