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. METHODOLOGY/PRINCIPAL FINDINGS: 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.

Serological response and persistence in schoolchildren with high baseline seropositive rate after receiving 2009 pandemic influenza A(H1N1) vaccine.

The serological response of the current 2009 H1N1 pandemic influenza monovalent vaccine in children exhibiting high baseline seropositive rate was evaluated though a community-based household study. Seroprotection rate of >90% and seroconversion rate of >50% were observed in children one month after receiving the pandemic vaccine. Among children with low baseline antibody titer, a significant lower seroconversion rate (55%) was observed in children who received seasonal trivalent inactivated vaccine (TIV) prior to pandemic vaccine, when compared with those receiving the pandemic vaccine only (86%). Persistence of antibody against the pandemic influenza virus was observed 6 months after vaccination in >80% of children presenting seroprotective antibody levels.