Vaccine 29 (2011) 617–623 Contents lists available at ScienceDirect Vaccine journal homepage: www.elsevier.com/locate/vaccine Short communication Serological response and persistence in schoolchildren with high baseline seropositive rate after receiving 2009 pandemic influenza A(H1N1) vaccine Day-Yu Chao a,∗,1 , Kuang-Fu Cheng b,1 , Ying-Hen Hsieh c , Tsai-Chung Li b,c , Trong-Neng Wu c,d , Chiu-Ying Chen d , Chen-An Tsai b,c , Jin-Hwa Chen b,c , Hsien-Tsai Chiu b,d , Jang-Jih Lu e,f , Mei-Chi Su f , Yu-Hsin Liao a , CIDER 2 a Graduate Institute of Microbiology and Public Health, College of Veterinary Medicine, National Chung-Hsing University, Taichung 401, Taiwan b Biostatistics Center, China Medical University, Taichung, Taiwan c Department of Public Health, China Medical University, Taichung, Taiwan d Graduate Institute of Biostatistics, China Medical University, Taichung, Taiwan e Graduate Institute of Clinical Medical Science, China Medical University, Taichung, Taiwan f Department of Laboratory Medicine, China Medical University Hospital, Taichung, Taiwan article info Article history: Received 4 October 2010 Received in revised form 1 November 2010 Accepted 6 November 2010 Available online 21 November 2010 Keywords: Pandemic influenza 2009 pH1N1 Vaccine Serology Immune response Schoolchildren abstract 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. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction In April 2009, a novel influenza A (H1N1) virus that is similar to the influenza viruses previously identified in swine was determined to be the cause of an influenza respiratory illness that spread across North America and was declared a worldwide pandemic by World Health Organization (WHO) in June [1–3]. The 2009 pandemic influenza A (pH1N1) virus contains a novel constellation of gene segments, which most likely stemmed from triple re-assortment of two or more viruses of swine, human, and avian origins [4]. Pre- vious serosurveys have demonstrated little or no cross-protection of the pediatric sera to the pH1N1 virus, which leaves the young ∗ Corresponding author at: Graduate Institute of Microbiology and Public Health, College of Veterinary Medicine, National Chung-Hsing University, Taichung 401, Taiwan. Tel.: +886 4 22840694; fax: +886 4 22852186. E-mail address: dychao@nchu.edu.tw (D.-Y. Chao). 1 These authors contributed equally to this work as first authors. 2 Center for Infectious Disease Education and Research, China Medical University, Taichung, Taiwan. children susceptible to infection [5]. For the coming influenza sea- son of 2010–2011 in the post-pandemic period, a safe and effective pH1N1 vaccine for children is urgently needed. The effectiveness of influenza vaccination in children, in reduc- ing infections and transmission among household members and the community, has been well documented [6–9]. Several model- ing analyses indicate that targeted mass immunization of children will contribute to the optimal control and prevention of pandemic and seasonal influenza [10–12]. During the past influenza season of 2009–2010, many governments and vaccine makers began to produce vaccines against the pH1N1 virus in massive quantity. The results from clinical trials reported in US, Europe, China, and Taiwan suggested that a good immunogenicity was generated after one or two doses of vaccine were administered [13–20]. However, the evaluation of antibody response after vaccination in community settings with high baseline seropositivity rate is still lacking. Moreover, in light of the inadequate cross-protection from either seasonal H1N1 (sH1N1) or pH1N1, the Advisory Commit- tee on Immunization Practice (ACIP) from United States Centers for Disease Control and Prevention (US-CDC) recommended the simul- 0264-410X/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.vaccine.2010.11.016