The aim of this study was to estimate the mucosal immune response induced by parenteral pneumococcal vaccines. How the response correlates with protection still remains unanswered. However, pneumococcal conjugate vaccines have already been shown to reduce nasopharyngeal carriage of the pneumococcal serotypes included in the vaccines (Dagan et al. 1996, Dagan et al. 1997), indicating mucosal protection. Some preliminary data also suggest protection against AOM (Black et al. 1999), but specific proof of the efficacy of the conjugate vaccines in preventing pneumococcal AOM still remains to be achieved. The Finnish Otitis Media Vaccine Trial (FinOM) was initiated in 1995, the target of which is to establish PncCRM vaccine's protection against culture-confirmed pneumococcal AOM. Results from this study are expected this year.
The exact role of mucosal S-IgA and of serum IgG in reduction of nasopharyngeal carriage and AOM also needs to be elucidated. Experimental studies have suggested a role for both S-IgA and IgG in preventing colonization and AOM. Human studies on passive immuno-prophylaxis have suggested that systemic PS-specific IgG may protect from pneumococcal AOM (Schurin et al. 1993). On the other hand, the presence of pathogen-specific anti-capsular S-IgA in the middle-ear fluid has been shown to be beneficial for resolution of AOM (Sloyer et al. 1974, Sloyer et al. 1976, Karjalainen et al.1990). The FinOM trial is a prospective study that includes extensive serological analysis, both of mucosal and systemic antibody responses. After the results from these studies are available, we can expect to have better estimates of serological correlations with protection against AOM.
Because mucosal protection can be expected to be at least partially S-IgA-mediated, the mucosal administration route of antigens is being intensively studied and can be expected to provide new strategies against microbes invading the body through mucosal surfaces. Furthermore, pneumococcal proteins: PspA, PsaA, and pneumolysin, and their ability to induce antibody-mediated protection, is currently a field of interest. Both mucosal and systemic protection are being evaluated, and mucosal administration of these proteins has been intensively studied. Animal studies on mucosal vaccination have also suggested that mucosal administration can sufficiently induce biologically active serum IgG responses (Flanagan et al. 1997, Seong et al. 1999, Jakobsen et al. 1999). If the same can be accomplished in human beings, systemic infections that do not involve invasion through mucosal surfaces (e.g., tetanus) could also be prevented by mucosal vaccination; some preliminary data on mucosal immunogenicity in humans already exists (Aggerbeck et al. 1997). Significant effort is being invested in research on mucosal vaccines, and the feasibility of vaccination via mucosal routes should be appraised in few years. An optimistic view for the future would be a single-dose vaccine given to small infants via the mucosal route.