Subjects with severe motor and intellectual disability (SMID) are considered to be debilitated and at high risk of influenza infection. However, the safety and immunogenicity of pandemic H1N1 (pH1N1) vaccine in these subjects have not been reported. We measured the hemagglutination inhibition antibody titer and calculated the geometric mean titer ratio (GMTR), seroprotection rate, and seroconversion rate in 104 subjects with SMID (mean age±standard deviation 40.1±12.9 years), and in 179 healthcare workers (40.7±10.4 years) in a long-term care facility. Antibody responses after the first dose of pH1N1 vaccine among workers were greater than the European Medicines Evaluation Agency criteria and US Food and Drug Administration (FDA) criteria: the seroprotection rate was 79.9% (95% confidence interval (CI) 73.3-85.5), the seroconversion rate was 77.9% (95%CI: 70.8-84.0), and GMTR was 7.3 (95%CI: 6.9-7.8). Responses among subjects with SMID were lower than the FDA criteria: the seroprotection rate was 56.3% (95%CI: 46.2-66.1), the seroconversion rate was 54.1% (95%CI: 43.7-64.2), and GMTR was 5.4 (95%CI: 4.9-5.9). Any additional antibody response induced by the second dose of vaccine among subjects with SMID was limited. Multivariate analysis indicated that subjects with SMID had a significantly lower seroprotection rate (odds ratio (OR) 0.37, 95%CI: 0.20-0.66) and seroconversion rate (OR 0.34, 95%CI: 0.20-0.59) than healthcare workers. No serious adverse reaction was reported in either group. These results indicate that a single dose of pH1N1 vaccine does not induce sufficient immunity among subjects with SMID, and a second dose is likely to be ineffective because of diminished immunogenicity. Further study is required to determine if vaccination over consecutive influenza seasons can improve immunogenicity in subjects with SMID.
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