Serological evidence of subclinical transmission of the 2009 pandemic H1N1 influenza virus outside of Mexico.
BACKGROUND
Relying on surveillance of clinical cases limits the ability to understand the full impact and severity of an epidemic, especially when subclinical cases are more likely to be present in the early stages. Little is known of the infection and transmissibility of the 2009 H1N1 pandemic influenza (pH1N1) virus outside of Mexico prior to clinical cases being reported, and of the knowledge pertaining to immunity and incidence of infection during April-June, which is essential for understanding the nature of viral transmissibility as well as for planning surveillance and intervention of future pandemics.


RESULTS
Starting in the fall of 2008, 306 persons from households with schoolchildren in central Taiwan were followed sequentially and serum samples were taken in three sampling periods for haemagglutination inhibition (HI) assay. Age-specific incidence rates were calculated based on seroconversion of antibodies to the pH1N1 virus with an HI titre of 1:40 or more during two periods: April-June and September-October in 2009. The earliest time period with HI titer greater than 40, as well as a four-fold increase of the neutralization titer, was during April 26-May 3. The incidence rates during the pre-epidemic phase (April-June) and the first wave (July-October) of the pandemic were 14.1% and 29.7%, respectively. The transmissibility of the pH1N1 virus during the early phase of the epidemic, as measured by the effective reproductive number R(0), was 1.16 (95% confidence interval (CI): 0.98-1.34).


CONCLUSIONS
Approximately one in every ten persons was infected with the 2009 pH1N1 virus during the pre-epidemic phase in April-June. The lack of age-pattern in seropositivity is unexpected, perhaps highlighting the importance of children as asymptomatic transmitters of influenza in households. Although without virological confirmation, our data raise the question of whether there was substantial pH1N1 transmission in Taiwan before June, when clinical cases were first detected by the surveillance network.
Graduate Institute of Microbiology and Public Health, College of Veterinary Medicine, National Chung-Hsing University, Taichung, Taiwan.
Day-Yu
DY
Chao
Kuang-Fu
KF
Cheng
Tsai-Chung
TC
Li
Trong-Neng
TN
Wu
Chiu-Ying
CY
Chen
Chen-An
CA
Tsai
Jin-Hua
JH
Chen
Jin-Hwa
JH
Chen
Hsien-Tsai
HT
Chiu
Jang-Jih
JJ
Lu
Mei-Chi
MC
Su
Yu-Hsin
YH
Liao
Wei-Cheng
WC
Chan
Ying-Hen
YH
Hsieh
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D002648:Child; D002675:Child, Preschool; D003429:Cross Reactions / Q000276:immunology; D004196:Disease Outbreaks; D005260:Female; D006385:Hemagglutination Inhibition Tests; D006801:Humans; D015994:Incidence; D053118:Influenza A Virus, H1N1 Subtype; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology / Q000635:transmission*; D008297:Male; D008800:Mexico / Q000453:epidemiology; D008875:Middle Aged; D058873:Pandemics; D012698:Serologic Tests; D013624:Taiwan; D055815:Young Adult
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