Dynamics of meningococcal long-term carriage among university students andtheir implications for mass vaccination

In the 1997-98 academic year, we conducted a longitudinal study of meningoc occal carriage and acquisition among first-year students at Nottingham Univ ersity, Nottingham, United Kingdom. We examined the dynamics of long-term m eningococcal carriage with detailed characterization of the isolates. Phary ngeal swabs were obtained from 2,453 first-year students at the start of th e academic year (October), later on during the autumn term, and again in Ma rch. Swabs were immediately cultured on selective media, and meningococci w ere identified and serologically characterized. Nongroupable strains were g enetically grouped using a PCR-based assay. Pulsed-field gel electrophoresi s was used to determine the link between sequential isolates. Of the carrie rs initially identified in October, 44.1% (98 of 222) were still positive l ater on in the autumn (November or December); 57.1% of these remained persi stent carriers at 6 months. Of the index carriers who lost carriage during the autumn, 16% were recolonized at 6 months. Of 344 index noncarriers foll owed up, 22.1% acquired carriage during the autumn term and another 13.7% a cquired carriage by March. Overall, 43.9% (397 of 904) of the isolates were noncapsulated (serologically nongroupable); by PCR-based genogrouping, a q uarter of these belonged to the capsular groups B and C. The ratio of capsu lated to noncapsulated forms for group B and C strains was 2.9 and 0.95, re spectively. Sequential isolates of persistent carriers revealed that indivi duals may carry the same or entirely different organisms at different times . We identified three strains that clearly switched their capsular expressi on on and off at different times in vivo. One student developed invasive me ningococcal disease after carrying the same organism for over 7 weeks. The study revealed a high rate of turnover of meningococcal carriage among stud ents. Noncapsulated organisms are capable of switching their capsular expre ssion on and off (both ways) in the nasopharynx, and group C strains are mo re likely to be noncapsulated than group B strains. Carriage of a particula r meningococcal strain does not necessarily protect against colonization or invasion by a homologous or heterologous strain.