Comparative studies on pharyngeal carriage of Neisseria meningitidis during a localized outbreak of serogroup C meningococcal disease.

An outbreak involving 20 cases of serogroup C meningococcal disease, predominantly among teenagers, occurred over a 7-month period in the Randers area of Denmark. The cases were caused by serogroup C:2a:P1.2 sulphonamide-resistant strains. The available evidence was against the transmission being related to particular schools. The outbreak was experienced as 3 clusters. At 2 schools involved in the first and the third cluster of the outbreak, 351 students were examined regarding pharyngeal carriage of meningococci, 282 of whom were tested again 17 weeks later; 308 students attending two similar schools in a nearby area were examined once. The majority of strains isolated from group C carriers in the high-risk area were serologically indistinguishable from the outbreak strain (13/14 = 95%), but less often sulphonamide-resistant (5/13 = 38%). In both areas, the overall rate (30%), the overall group C rate (3%), the carrier rate for the outbreak strain (1%) were the same. The attack rate for the outbreak strain differed significantly: 1/40 in the high-risk area versus 1/2,500 in the normal risk area. No conditions that might explain this difference were revealed. Immediately after recognition of the first and the third cluster, 780 and 13,300 students, respectively, were vaccinated with meningococcal polysaccharide vaccine A+C. It was concluded that the definition of target groups for vaccination should be liberal, because the "at risk" population may be difficult to recognize at the onset of an outbreak.