The infection attack rate and severity of 2009 pandemic H1N1 influenza in Hong Kong.
BACKGROUND
Serial cross-sectional data on antibody levels to the 2009 pandemic H1N1 influenza A virus from a population can be used to estimate the infection attack rates and immunity against future infection in the community.


METHODS
From April through December 2009, we obtained 12,217 serum specimens from blood donors (aged 16-59 years), 2520 specimens from hospital outpatients (aged 5-59 years), and 917 specimens from subjects involved in a community pediatric cohort study (aged 5-14 years). We estimated infection attack rates by comparing the proportions of specimens with antibody titers ≥ 1:40 by viral microneutralization before and after the first wave of the pandemic. Estimates were validated using paired serum samples from 324 individuals that spanned the first wave. Combining these estimates with epidemiologic surveillance data, we calculated the proportion of infections that led to hospitalization, admission to the intensive care unit (ICU), and death.


RESULTS
We found that 3.3% and 14% of persons aged 5-59 years had antibody titers ≥ 1:40 before and after the first wave, respectively. The overall attack rate was 10.7%, with age stratification as follows: 43.4% in persons aged 5-14 years, 15.8% in persons aged 15-19 years, 11.8% in persons aged 20-29 years, and 4%-4.6% in persons aged 30-59 years. Case-hospitalization rates were 0.47%-0.87% among persons aged 5-59 years. Case-ICU rates were 7.9 cases per 100,000 infections in persons aged 5-14 years and 75 cases per 100,000 infections in persons aged 50-59 years, respectively. Case-fatality rates were 0.4 cases per 100,000 infections in persons aged 5-14 years and 26.5 cases per 100,000 infections in persons aged 50-59 years, respectively.


CONCLUSIONS
Almost half of all school-aged children in Hong Kong were infected during the first wave. Compared with school children aged 5-14 years, older adults aged 50-59 years had 9.5 and 66 times higher risks of ICU admission and death if infected, respectively.
Wu
Joseph T
JT
Department of Community Medicine and School of Public Health, Li Ka Shing Faculty of Medicine, The University of Hong Kong, People's Republic of China. joewu@hku.hk
Ma
Edward S K
ES
Lee
Cheuk Kwong
CK
Chu
Daniel K W
DK
Ho
Po-Lai
PL
Shen
Angela L
AL
Ho
Andrew
A
Hung
Ivan F N
IF
Riley
Steven
S
Ho
Lai Ming
LM
Lin
Che Kit
CK
Tsang
Thomas
T
Lo
Su-Vui
SV
Lau
Yu-Lung
YL
Leung
Gabriel M
GM
Cowling
Benjamin J
BJ
Malik Peiris
J S
JS
D000293:Adolescent; D000328:Adult; D000914:Antibodies, Viral / Q000097:blood*; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D004196:Disease Outbreaks / Q000706:statistics & numerical data*; D006723:Hong Kong / Q000453:epidemiology; D006801:Humans; D007074:Immunoglobulin G / Q000097:blood*; D053118:Influenza A Virus, H1N1 Subtype / Q000276:immunology*; D007251:Influenza, Human / Q000453:epidemiology* / Q000276:immunology / Q000821:virology*; D008390:Markov Chains; D008875:Middle Aged; D009010:Monte Carlo Method; D009500:Neutralization Tests; D015203:Reproducibility of Results; D016036:Seroepidemiologic Studies
D000914:Antibodies, Viral; D007074:Immunoglobulin G