Results of a study on the under-detection of meningococcus in vaccinated subjects in Galicia

BACKGROUND: On the basis of active surveillance and the monitoring of Meningococcal Disease (MD) following the vaccination campaign carried out in Galicia, it was observed that the proportion of isolations of the serogroups responsible for the disease among individuals suspected of Meningococcal Disease (SMD) who had been vaccinated was lower than among unvaccinated individuals. In view of this situation, a study was made in order to determine whether in the origin of those SMDs that were not isolated, we would find N. Meningitidis serogroup C, and to quantify the significance of the sub-detection of same. METHODS: For this purpose, and during the period under study (from the 26th week of 1997 to the 14th week of 1999), blood and cephalorachidian fluid samples were taken from the SMDs without isolation for their study with C protein reagent for type and serogroup. The analysis of the samples was performed by the microbiology laboratory of the Clinical Hospital of Santiago de Compostela. RESULTS: Of the 120 cases notified during the period under study, 65 were analysed by C protein reagent (38 vaccinated and 27 unvaccinated), with a positive reading for N. meningitidis in 65% (42 samples) 74% in vaccinated individuals and 52% in unvaccinated. By estimating, on the basis of the cases studied, the results for the total, and excluding the C protein reagent negative cases, we find that, for serogroup C, in only 27% of the cases occurring in vaccinated individuals was it possible to isolate it, in comparison with 80% in the case of unvaccinated subjects (p < 0.0001). These percentages are, in the case of serogroup B, 59% and 71%, respectively, a difference which is not statistically significant. CONCLUSIONS: The vaccine brought about an true sub-detection of serogroup C meningococci in the vaccinated cases.