title: Evaluation and validation of a serum bactericidal antibody assay for Haemophilus influenzae type b and the threshold of protection.
abstract: Prior to routine immunisation, Haemophilus influenzae serotype b (Hib) was a major cause of serious bacterial infections, particularly in young children. In the United Kingdom, introduction of the Hib conjugate vaccine into the national childhood immunisation schedule has led to a sustained decline in invasive Hib disease across all age-groups. Evaluation of the immune response to Hib conjugate vaccines involves measurement of serum IgG antibodies against the capsular polyribosyl-ribitol-phosphate (PRP) polysaccharide by enzyme-linked immunosorbent assay (ELISA), with accepted short-term and long-term protective thresholds of ≥0.15μg/mL and ≥1.0μg/mL, respectively. These levels were derived by passive immunisation or immunisation with pure polysaccharide, and their relevance for protection following immunisation with conjugate vaccines remains unclear. This study aimed to modify and optimise a serum bactericidal antibody (SBA) assay to evaluate the functional activity of Hib antibodies generated following Hib conjugate vaccination. Validation of the Hib SBA assay was deemed acceptable for all assay parameters tested. A strong correlation between anti-PRP IgG concentrations and SBA titres was observed in vaccinated adults (r=0.81), as well as infants after primary immunisation at 2, 3, and 4 months (r=0.635) and after the 12-month booster (r=0.746). The assay identified some children with high anti-PRP IgG but low SBA activity and vice versa. The predictive protective SBA titre corresponding to a post-booster anti-PRP IgG of 1.0μg/mL was 8. Thus, the optimised Hib SBA assay was specific and reproducible and correlated with anti-PRP IgG. Such assays may have a role in evaluating immune responses to conjugate vaccines in addition to measuring capsular antibodies.
authors.affiliation: Vaccine Evaluation Unit, Public Health England, Clinical Sciences Building, Manchester Royal Infirmary, Manchester, UK.
authors.forename: Kelly
authors.initials: K
authors.lastname: Townsend
authors.affiliation: Immunisation, Hepatitis and Blood Safety Department, Centre for Infectious Disease Surveillance and Control, Public Health England, Colindale, London, UK. Electronic address: shamez.ladhani@phe.gov.uk.
authors.forename: Shamez N
authors.initials: SN
authors.lastname: Ladhani
authors.affiliation: Vaccine Evaluation Unit, Public Health England, Clinical Sciences Building, Manchester Royal Infirmary, Manchester, UK.
authors.forename: Helen
authors.initials: H
authors.lastname: Findlow
authors.affiliation: Vaccine Evaluation Unit, Public Health England, Clinical Sciences Building, Manchester Royal Infirmary, Manchester, UK; Inflammation Sciences Research Group, University of Manchester, School of Translational Medicine, Stopford Building, Manchester, UK.
authors.forename: Ray
authors.initials: R
authors.lastname: Borrow
mesh_terms: D000328:Adult; D000907:Antibodies, Bacterial / Q000097:blood*; D016667:Bacterial Capsules; D006192:Haemophilus Infections / Q000517:prevention & control; D018073:Haemophilus Vaccines / Q000627:therapeutic use*; D019741:Haemophilus influenzae type b; D006801:Humans; D007117:Immunization, Secondary; D007074:Immunoglobulin G / Q000097:blood; D007223:Infant; D012680:Sensitivity and Specificity; D058248:Serum Bactericidal Antibody Assay / Q000379:methods* / Q000592:standards
chemical_list: D000907:Antibodies, Bacterial; D018073:Haemophilus Vaccines; C044237:Haemophilus influenzae type b polysaccharide vaccine; D007074:Immunoglobulin G
keywords: Correlate of protection; Haemophilus influenzae type b; Serum bactericidal antibody