2009 influenza A(H1N1) seroconversion rates and risk factors among distinct adult cohorts in Singapore.
BACKGROUND
Singapore experienced a single epidemic wave of 2009 influenza A(H1N1) with epidemic activity starting in late June 2009 and peaking in early August before subsiding within a month.


OBJECTIVE
To compare the risk and factors associated with H1N1 seroconversion in different adult cohorts.


METHODS
A study with serial serological samples from 4 distinct cohorts: general population (n = 838), military personnel (n = 1213), staff from an acute care hospital (n = 558), and staff as well as residents from long-term care facilities (n = 300) from June 22, 2009, to October 15, 2009. Hemagglutination inhibition results of serum samples taken before, during, and after the epidemic and data from symptom questionnaires are presented.


METHODS
A 4-fold or greater increase in titer between any of the 3 serological samples was defined as evidence of H1N1 seroconversion.


RESULTS
Baseline titers of 40 or more were observed in 22 members (2.6%; 95% confidence interval [CI], 1.7%-3.9%) of the community, 114 military personnel (9.4%; 95% CI, 7.9%-11.2%), 37 hospital staff (6.6%; 95% CI, 4.8%-9.0%), and 20 participants from long-term care facilities (6.7%; 95% CI, 4.4%-10.1%). In participants with 1 or more follow-up serum samples, 312 military personnel (29.4%; 95% CI, 26.8%-32.2%) seroconverted compared with 98 community members (13.5%; 95% CI, 11.2%-16.2%), 35 hospital staff (6.5%; 95% CI, 4.7%-8.9%), and only 3 long-term care participants (1.2%; 95% CI, 0.4%-3.5%). Increased frequency of seroconversion was observed for community participants from households in which 1 other member seroconverted (adjusted odds ratio [OR], 3.32; 95% CI, 1.50-7.33), whereas older age was associated with reduced odds of seroconversion (adjusted OR, 0.77 per 10 years; 95% CI, 0.64-0.93). Higher baseline titers were associated with decreased frequency of seroconversion in community (adjusted OR for every doubling of baseline titer, 0.48; 95% CI, 0.27-0.85), military (adjusted OR, 0.71; 95% CI, 0.61-0.81), and hospital staff cohorts (adjusted OR, 0.50; 95% CI, 0.26-0.93).


CONCLUSIONS
Following the June-September 2009 wave of 2009 influenza A(H1N1), 13% of the community participants seroconverted, and most of the adult population likely remained susceptible.
Chen
Mark I C
MI
Tan Tock Seng Hospital, Department of Clinical Epidemiology, 11 Jalan Tan Tock Seng, Singapore, 308433. mark_chen@ttsh.com.sg
Lee
Vernon J M
VJ
Lim
Wei-Yen
WY
Barr
Ian G
IG
Lin
Raymond T P
RT
Koh
Gerald C H
GC
Yap
Jonathan
J
Cui
Lin
L
Cook
Alex R
AR
Laurie
Karen
K
Tan
Linda W L
LW
Tan
Boon Huan
BH
Loh
Jimmy
J
Shaw
Robert
R
Durrant
Chris
C
Chow
Vincent T K
VT
Kelso
Anne
A
Chia
Kee Seng
KS
Leo
Yee Sin
YS
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000914:Antibodies, Viral; D000917:Antibody Formation; D015331:Cohort Studies; D004198:Disease Susceptibility; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology / Q000276:immunology*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors; D012846:Singapore / Q000453:epidemiology; D055815:Young Adult
D000914:Antibodies, Viral