Dynamics of the meningococcal carrier state and characteristics of the carrier strains: a longitudinal study within three cohorts of military recruits.

Three cohorts of Danish male military recruits (n = 1069) were studied for pharyngeal meningococcal carriage during 3 months at different seasons: 39-47% of entrants were meningococcal carriers and the carriage rate remained constant over time and season. However, individual changes in the carrier state occurred frequently, and after 3 months 34% had changed carrier state on one or more occasions. Initially, a loss of carriage predominated; on the other hand almost 20% of non-carriers had acquisition of meningococci within the first month. The serological phenotypes of the 670 carrier strains were compared with those of 261 invasive strains recovered concurrently from patients with meningococcal disease country-wide. Both carrier strains and invasive strains were phenotypically heterogeneous. Almost 60% of the invasive strains belonged to three phenotypes: B:15:P1.7, 16, C:2a:P1.2, 5 and C:2b:P1.2, 5. In contrast, these phenotypes only amounted to 3.2% of the carrier strains, among which no phenotype was found with a prevalence above 4.9%. However, 30% of the carrier strains had serological phenotypes identical to those of 80% of the invasive strains. Our results indicated that the transmission rate of potential pathogenic carrier strains did not differ from that of other carrier strains.