Haemophilus influenzae type b (Hib) causes several invasive infections such as meningitis, septic arthritis, and pneumonia, especially in children below 5 years old. Despite the availability of Hib vaccines against Hib infection, seroepidemiological surveys of Hib infections have not yet been systematically conducted in Japan. We analyzed 1,338 serum samples, provided by the National Serum Reference Bank of the National Institute of Infectious Diseases (Tokyo, Japan), from 0- to 5-year-old children. Anti-polyribosylribitol phosphate (PRP) immunoglobulin G (IgG) antibody levels against Hib were determined using an anti-H. influenzae IgG enzyme immunoassay kit. In a total of 1,168 (87.3%) serum samples from children, anti-PRP IgG antibody levels were ≥ 0.15 μg/mL, providing natural immunity. Titers expected to provide long-term protection (≥ 1 μg/mL) were increased from 2.7% to 51.6% in children < 1 year old after the introduction of Hib vaccine (1980, 5.3%; 1995, 2.7%; 2010, 22%; 2012, 51.6%). Our data confirmed that the introduction of Hib vaccination in children below 5 years old increased the proportion of children having high anti-PRP IgG antibody levels, ensuring long-term protection against Hib.
Keywords: Haemophilus influenzae; Japan; vaccination.