Re-evaluation of the immunogenicity of meningococcal serogroups A and C polysaccharide vaccine among a healthy population aged 5 to 59

Objective To re-evaluate the immunogenicity of meningococcal serogroups A and C polysaccharide vaccine among a healthy population of age 5 to 59.Methods Pre and post-vaccination ser-um samples were collected from the subjects involved in a randomized , controlled clinical trial conducted in 2005 at Hechi, Guangxi, China.Enzyme-linked immunosorbent assay （ELISA） was performed for the quan-titative detection of specific anti-capsule IgG antibody against meningococcal serogroups A and C in serum samples.Serum bactericidal assay （ SBA） was used to measure the bactericidal antibody activity against se-rougroups A and C bacterial strains .Results Geometric mean concentrations （GMCs） of specific anti-cap-sule IgG antibody were 23.66 μg/ml and 78.83 μg/ml for serogroup A （P〈0.05）, and 1.23 μg/ml and 51.25 μg/ml for serogroup C （P〈0.05） in serum samples of pre and post-vaccination, respectively.Be-fore immunization , 99% of serum samples （ 97/98 ） showed serogroup A-specific IgG concentration ≥2μg/ml, which reached up to 100%（98/98） after vaccination （P〉0.05）.The percentage of serum sam-ples with serogroup C-specific IgG concentration ≥2 μg/ml rose from 20%to 99% after vaccination （ P〈0.05）.The ratio of positive serum samples with at least four times of increases in concentration of specific IgG against serogroup A and serogroup C after vaccine immunization were 34% and 100%, respectively . The rSBA geometric mean titers （ GMTs） for serogroups A and C in serum sample of post-vaccination were＆amp;nbsp;respectively elevated from 1164 to 5530 （P〈0.05）, and from 4 to 6225 （P〈0.05）.The percentage of se-rum samples with rSBA GMTs≥128 increased from 91% to 99% for serogroup A （P〉0.05）, and from 14%to 96%for serogroup C （P〈0.05） after vaccination.The rSBA GMTs with at least four times of in-creases after vaccination were detected respectively in 53% and 100% of serogroups A and C vaccinated subjects.Pearson correlation coefficient between IgG concentration and rSBA GMTs was r=0.15 （pre） and r=0.23 （post） for serogroup A, and r=-0.14 （pre） and r=0.58 （post） for serogroup C （P〈0.05）. Conclusion This study has demonstrated an efficient and sustained immunogenicity with meningococcal sero -groups A and C polysaccharide vaccine as evidenced by the data from standardized assays of ELISA and SBA . Key words: Meningococcal serogroups A and C polysaccharide vaccine; Immuogenicity