Porin A (PorA), which determines the serosubtype of Neisseria meningitidis, is the main antigen of a candidate vaccine against serogroup B meningococci,which has been shown to induce high-avidity antibodies in children. We characterized the immune response of children after convalescing from meningococcal infection with a serosubtype P1.7-2,4 strain. Acute-and convalescent-phase sera of 21 c hildren with meningococcal septic shock caused by strains with PorA subtype P 1. 7-2,4 were collected. The serum bactericidal antibody titers, IgG isotype distr ibution, and antibody avidity were measured. We determined whether the differenc es in avidity of anti-outermembrane vesicle antibodies were PorA specific. Seru m bactericidal activity against H44/76 P 1.7-2,4 was < 4 in all convalescent sera. The IgG i sotype distribution of the convalescent sera was dominated by IgG1, followed by IgG3 , whereas no IgG2 or IgG4 was found. The geometric mean avidity index (GMAI ) of convalescent sera measured against a strain with the identical subtype as t he infective isolate was significantly higher than that against a strain with a heterologous PorA subtype or a PorA negative mutant strain (57 versus 35 and 23 %, respectively; p = 0.005 and p < 0.001). Geometric mean avidity titers were h ighest for P1.7-2,4, corresponding with the highest GMAI. The GMAI after invasi ve meningococcal disease was lower than after vaccination of healthy toddlers wi th a monovalent P1.7-2,4 outer membrane vesicle vaccine.
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