Background: The current influenza pandemic calls for a safe and effective vaccine. We assessed the safety and immunogenicity of eight formulations of 2009 pandemic influenza A H1N1 vaccine produced by ten Chinese manufacturers. Methods: In this multicentre, double-blind, randomised trial, 12 691 people aged 3 years or older were recruited in ten centres in China. In each centre, participants were stratified by age and randomly assigned by a random number table to receive one of several vaccine formulations or placebo. The study assessed eight formulations: split-virion formulation containing 7·5 μg, 15 μg, or 30 μg haemagglutinin per dose, with or without aluminium hydroxide adjuvant, and whole-virion formulation containing 5 μg or 10 μg haemagglutinin per dose, with adjuvant. All formulations were produced from the reassortant strain X-179A (A/California/07/2009-A/PR/8/34). We analysed the safety (adverse events), immunogenicity (geometric mean titre [GMT] of haemagglutination inhibition antibody), and seroprotection (GMT ≥1:40) of the formulations. Analysis was by per protocol. Two sites registered their trial with ClinicalTrials.gov, numbers NCT00956111 and NCT00975572. The other eight studies were registered with the State Food and Drug Administration of China. Findings: 12 691 participants received the first dose on day 0, and 12 348 participants received the second dose on day 21. The seroprotection rate 21 days after the first dose of vaccine ranged from 69·5% (95% CI 65·9-72·8) for the 7·5 μg adjuvant split-virion formulation to 92·8% (91·9-93·6) for the 30 μg non-adjuvant split-virion formulation. The seroprotection rate was 86·5% (796 of 920; 84·1-88·7) in recipients of one dose of the 7·5 μg non-adjuvant split-virion vaccine compared with 9·8% (140 of 1432; 8·3-11·4) in recipients of placebo (p<0·0001). One dose of the 7·5 μg non-adjuvant split-virion vaccine induced seroprotection in 178 of 232 children (aged 3 years to <12 years; 76·7%, 70·7-82·0), 211 of 218 adolescents (12 years to <18 years; 96·8%, 93·5-98·7), 289 of 323 adults (18-60 years; 89·5%, 85·6-92·6), and 118 of 147 adults older than 60 years (80·3%, 72·9-86·4), meeting the European Union's licensure criteria for seroprotection in all age-groups. In children, a second dose of the 7·5 μg formulation increased the seroprotection rate to 97·7% (215 of 220, 94·8-99·3). Adverse reactions were mostly mild or moderate, and self-limited. Severe adverse effects occurred in 69 (0·6%, 0·5-0·8) recipients of vaccine compared with one recipient (0·1%, 0-0·2) of placebo. The most common severe adverse reaction was fever, which occurred in 25 (0·22%; 0·14-0·33) recipients of vaccine after the first dose and four (0·04%; 0·01-0·09) recipients of vaccine after the second dose compared with no recipients of placebo after either dose. Interpretation: One dose of non-adjuvant split-virion vaccine containing 7·5 μg haemagglutinin could be promoted as the formulation of choice against 2009 pandemic influenza A H1N1 for people aged 12 years or older. In children (aged <12 years), two 7·5 μg doses might be needed. Funding: Sinovac Biotech, Hualan Biological Bacterin, China National Biotec Group, Beijing Tiantan Biological Products, Changchun Institute of Biological Products, Changchun Changsheng Life Sciences, Jiangsu Yanshen Biological Technology Stock, Zhejiang Tianyuan Bio-Pharmaceutical, Lanzhou Institute of Biological Products, Shanghai Institute of Biological Products, and Dalian Aleph Biomedical. © 2010 Elsevier Ltd. All rights reserved.