Outbreaks of Meningococcal Disease in Schools
By Robert S. Baltimore, MD
A 50-state active-surveillance study was conducted by the Centers for Disease Control and Prevention (CDC).1 State health departments gathered information on outbreaks of meningococcal disease in schools that occurred from January 1989 to June 1994. The stimulus for this study was that standards for identification of such outbreaks, definition of individuals at increased risk, and appropriate control measures have not been well-defined. Strategies that have been employed in the past have varied. Twenty-two clusters in 15 states were identified. A case of meningococcal disease was defined as a presentation consistent with sepsis or meningitis and identification of the organism or meningococcal antigen in the blood or spinal fluid. A cluster was defined as two or more cases in children grades K-12 in which the cases occurred within 1-30 days of the first case.
The incidence of meningococcal disease in school-children 5-18 years of age was 2.5/100,000 and the relative risk compared with the population at large was 2.3. The median number of cases per cluster was two (range, 2-4). Secondary schools (grades 7-12) accounted for 75% of clusters. Thirty-three percent of secondary cases occurred within two days of the primary case and 73% of subsequent cases occurred within two weeks of the index case. In 73% of the clusters, the index case and secondary case(s) had interactions, including shared classrooms, school-bus rides, trips, assemblies, or dances. In four of these clusters, the contact included shared drinking cups or water bottles. Of the 20 clusters for which information was available, chemoprophylaxis with rifampin was used in 90%.
For a case-control analysis of risk factors for secondary cases, schools with secondary cases were compared with schools with a single case of meningococcal disease and no evidence of spread. The only significant difference was that in schools with clusters, the index case was more likely to have participated in a large student event such as an assembly, dance, or field trip. School size or characteristics of the physical plant were not associated with clusters.
It is well-known that close contacts of cases of meningococcal disease have a higher rate of meningococcal disease than the general population. Emphasis in the past has been concerned almost exclusively on family contacts (500- to 1000-fold higher rate than the general population) and prophylaxis has routinely been recommended for family members, and only in exceptional cases for others. These exceptions have included day care attendees, sleep-overs, and other examples of prolonged contact in family-like situations. It has been shown that the age-specific rate of secondary cases is similar to sporadic cases and that infants, toddlers, and younger children are at the highest risk.
Outbreaks in schools have received little attention in the past because cases appear to be sporadic despite the community panic that often accompanies the kind of dramatic presentation of meningococcal disease that newspapers often headline. The most appropriate management of cases in secondary schools has never been clearly stated and most recommendations come from the authors of studies on single outbreaks who have not had the benefit of a large analysis such as the one presented here.
This CDC-sponsored study demonstrates the advantages of large-scale data collection and careful epidemiological analysis. No individual or medical center could have sufficient experience with such uncommon events to determine whether there really is a significant risk of attending a school in which there has been a case of meningococcal disease and what the risk factors for spread would be. Indeed, a relative risk of 2.3 compared with the total aged-matched population is not exceedingly high and suggests the chance of acquiring meningococcal disease in school is actually quite a bit lower than previous estimates and accounts for only 1.2% of meningococcal disease cases in this age group.
The recommendations proposed for infection control appear reasonable. Meningococcal prophylaxis may be effected either through vaccination or chemoprophylaxis. Vaccination with the available quadrivalent vaccine which contains the capsular polysaccharide antigens of Sero groups A, C, Y, and W-135 does not provide prevention of infection due to Serogroup B meningococci, which accounts for 32% of cases and requires 7-10 days before protective immunity develops. Chemoprophylaxis eradicates respiratory tract carriage and provides rapid protection against all serogroups. The authors recommend that mass prophylaxis after a single case is not warranted because the overall risk is low and a high-risk population within the school can not be identified. They do recommend school-wide chemoprophylaxis with rifampin if a second case occurs. Such a policy could still prevent half of the secondary cases (the third and fourth cases) within the school. Based upon the authors’ data, this would affect only 4-8 schools per year in the United States.
1. Zangwill KM, et al. School-based clusters of meningococcal disease in the United States: Descriptive epidemiology and a case-control analysis. JAMA 1997;227:389-395.