Immunologic memory with no detectable bactericidal antibody response to a first dose of meningococcal serogroup C conjugate vaccine at four years

Fourteen children with no detectable bactericidal antibody response to a first dose of meningococcal C conjugate vaccine at 4 years of age were given a booster dose of the same vaccine 2 years later. A rapid 1000-fold rise in postimmunization bactericidal antibody titers, as measured either 7 or 14 days later, suggested previous immunologic priming.

Conjugate vaccines against Neisseria meningitidis serogroup C (Men C) were introduced into the routine infant immunization schedule of the UK in November 1999. In a concurrent “catch-up” campaign, infants between 6 and 12 months of age were given two doses of the vaccine, and a single dose was offered to all remaining children and adolescents. Few studies have addressed the immunogenicity of a single dose of vaccine in children of preschool or primary school age, or response to a booster dose. ¹

We studied 14 children who did not develop demonstrable bactericidal activity against the meningococcus after a first dose of Men C vaccine administered as part of a Phase II study at 4 years of age. ² The primary aim of the study was to look for evidence of previous priming for immunologic memory, on the basis of the magnitude and kinetics of the serum bactericidal assay (SBA) response to a second dose of Men C vaccine at 6 years of age. Secondary endpoints included measurement of Men C-specific IgG antibody using high avidity enzyme-linked immunosorbent assay (ELISA) after the booster dose and also preimmunization antibody (SBA and ELISA) to look for acquisition of immunity in the 2 years between immunizations. The safety of a booster dose at 6 years of age was also evaluated.

Materials and methods.

Subjects were identified on the basis of their prior participation in a Phase II study performed by the Oxford Vaccine Group to evaluate the immunogenicity and safety of Chiron Men C conjugate vaccine in preschool children. ² At the completion of that study, 21 of 107 (19.6%) first time recipients of the vaccine from whom postimmunization blood samples were obtained had no demonstrable bactericidal antibody response to Men C in a 1/4 dilution of serum. These 21 children were offered a booster dose of vaccine at 6 years of age, and their parents were approached to take part in a study of the immunogenicity and safety of this intervention.

Written informed consent to participation was taken from a parent or legal guardian before enrollment. Each child received one dose of licensed Chiron Men C conjugate vaccine (Menjugate) at the first study visit. A baseline blood sample was collected immediately before immunization. Children were randomized to have a second blood sample taken either 7 or 14 days later. Immunogenicity was assessed by measuring the specific serum antibody response to Men C in an SBA using a human complement source and by a modified ELISA designed to evaluate high avidity IgG antibody, as used in the study of 4-year-olds. ² For safety evaluation parents were asked to complete a diary card to describe local and systemic reactions occurring during the 7 days after vaccination. Axillary temperature was also recorded daily. All other adverse events experienced between the first and second visits were also recorded on this card.

Paired t tests were used to assess the significance of the rise in antibody titers for the groups bled at 7 and 14 days, separately. An unpaired t test was used to compare postimmunization titers between the two groups. All statistical analyses were performed with STATA 7.0. The necessary limitations of sample size made this study primarily descriptive, particularly when considering reactogenicity data.

Results.

Of the 21 eligible children, 14 (66.7%) were enrolled in the study. Six of the remainder received a booster dose of vaccine (2 through their General Practice surgeries), and one declined further immunization. Postimmunization reaction diaries were not completed by the parents of these children, but no adverse events were reported. Seven subjects were randomly assigned to each of the 7-day and 14-day blood sample groups. One child in the 7-day group withdrew consent before the second visit and thus contributed no postvaccination immunogenicity or safety data. Another child in this group was unavailable on the scheduled day and was bled on Day 14.

Immunogenicity results are shown in Table 1. Only four children (28.6%) had developed measurable bactericidal activity, as documented at baseline, in the interval between immunizations. All achieved approximately a 1000-fold increase in SBA and ELISA titers from baseline after immunization, whether blood was collected at 7 or at 14 days. This rise was significant (P < 0.0001) for both groups. No significant difference could be found between Day 7 and Day 14 values (P = 0.34), although this study had only 18% power to demonstrate divergence of results with 95% confidence given the sample size.

TABLE 1: Immunogenicity results

Local reactions to the vaccine were relatively common. Eight children (61.5%) described some tenderness at the injection site (one severe). Eight (61.5%) had an erythematous reaction, 3 (23.1%) between 1 and 25 mm, and 5 (38.5%) >50 mm in widest diameter. One child experienced erythema extending from shoulder to elbow within 48 h of vaccination that resolved completely within 3 days. No similar side effects had been reported after previous immunizations. Induration was present in 6 cases (46.2%), the involved area measuring 26 to 50 mm in one child and >50 mm diameter in one other. Systemic reactions to the vaccine were less frequent, with those most often reported among the 13 children being irritability [3 (23.1%)], anorexia [2 (15.4%)] and drowsiness [1 (7.7%)]. One child (7.7%) was febrile (>38°C), and 3 children (23.1%) had symptoms sufficient to require paracetamol. No gastrointestinal or skin reactions were recorded. No serious adverse events occurred.

Discussion.

All the children given a booster dose of Men C vaccine in this study mounted a specific bactericidal antibody response to meningococcus serogroup C. Although no age-matched control group was available for comparison, the titers observed were 100 times greater than those seen after a first dose of vaccine at 4 years of age, ² consistent with an anamnestic response. This was reassuring, confirming that priming for immunologic memory had occurred, most likely attributable to the first dose of vaccine, even in the absence of apparent seroconversion with a functional assay. Although 4 children had some measurable bactericidal antibody activity suggesting stimulation of immunity by a carriage event, it would be unlikely for all 13 to have had natural priming exposures during a 2-year period. Further, all had made some detectable IgG (ELISA) response after the preschool dose. Such findings are consistent with a study performed by the Public Health Laboratory Service in toddlers 17 to 28 months old with absent or equivocal antibody responses after a single dose of Men C conjugate vaccine. Of those given a booster dose of 10 μg of plain Men C polysaccharide vaccine, 90% seroconverted, in contrast to the 10% who would be expected to respond to a primary dose of polysaccharide vaccine at this age. ³ Similarly infants without a measurable immune response after a primary infant course of conjugate Haemophilus influenzae type b (Hib) vaccine combined with diphtheria, tetanus and acellular pertussis had evidence of immunologic memory after boosting with the plain polysaccharide polyribosylribitol phosphate 1 year later. ⁴

No significant difference was observed between the magnitudes of responses detected at 7 and 14 days with either SBA or ELISA, although repeated measurements were not available from individual subjects and sample size was extremely limited. Whether or not a further increase occurred in the second week postimmunization, very high antibody titers were achieved within the first week. Previous work in adults has shown rising bactericidal antibody concentrations within 7 days of documented meningococcal colonization ⁵ and somewhere between 4 and 10 days after immunization with the Men C conjugate vaccine. ⁶ Similar kinetics have been observed after booster doses of Hib vaccines in toddlers. ⁷ This rapidity of memory immune responses may be important in preventing progression to invasive disease after contact with the organism. Variation in either the quality or timing of recall responses or invasiveness of the pathogen may, however, leave some individuals vulnerable for a critical period. Evidence for this comes from the observation that Hib meningitis may develop in spite of rapidly rising specific antibody titers against the Hib capsular polysaccharide. ⁸

The incidence of tenderness at the injection site and experience of systemic symptoms after immunization were similar to those observed at 4 years. ² The occurrence of erythema and induration at the injection site was approximately twice that seen in the previous study, ² or in response to administration of a first dose of Men C to adolescents. ⁹ It was, however, very similar to the frequency described after the preschool diphtheria-tetanus toxoids (DT) booster ¹⁰ and may have reflected the fact that these children had received three doses of diphtheria, two as Men C and one as DT, within a period of two years. Given the small sample size, however, it is difficult to extrapolate from these data the expected frequency of the more severe reactions described.

Can we assume that the children in this study were protected by their first vaccination, given that immunologic memory had been induced in the absence of initial seroconversion? Similarly how efficacious will memory be at preventing Men C infections in children in whom postimmunization antibody titers have declined? The only proven correlate of protection against invasion with this organism is the presence of resting bactericidal activity in a 1/4 serum dilution, measured in an assay with the use of a human complement source. ¹¹ Whether demonstration of such a titer only in response to a booster dose of vaccine will predict efficacy remains to be seen. Population experience with Hib conjugate vaccines in the UK has demonstrated high vaccine effectiveness, in spite of rapidly waning immunity, following an accelerated infant primary schedule without a booster dose. ¹² This success has been attributed to a combination of immunologic memory and the herd immune effect. Doubts regarding the longevity of this impact have been raised more recently, however, by a steady increase since 1999 in the incidence of Hib vaccine failure after full primary infant immunization. ¹³ Short term estimates of Men C vaccine efficacy in the UK are encouraging, ranging from 80% for a partial infant primary course to 100% for a single dose in toddlers. ¹⁴ The importance of long term surveillance to continue to monitor the effectiveness of the current immunization schedule, which must protect well into the second decade of life, is clear.

Acknowledgments.

We thank all of the children who took part in the study, and their parents. We also thank Linda Diggle, Tracey Chantler, Marion Archer and Karen Stone for assistance in conducting visits. This study was supported by a grant from Chiron Vaccines, Siena, Italy.