A retrospective survey of clusters of meningococcal disease in England and Wales, 1993 to 1995: estimated risks of further cases in household and educational settings.

Information about the epidemiology of meningococcal disease case clusters and the risk of further cases is sparse. Data on clusters in household and educational settings from 1 January 1993 to 31 March 1995 was requested from consultants in communicable disease control in England and Wales through a retrospective postal survey. Ninety-three per cent (122/131) responded. Of the 114 cases in 45 reported clusters, 77 (67.5%) were microbiologically confirmed. The case fatality rate in index cases was higher than in associated cases (18.2% vs 4.5%; p = 0.02). Five out of 11 clusters in household settings consisted only of index and co-primary cases. No further cases occurred within two weeks after giving chemoprophylaxis to household contacts. The relative risks of further cases in the week after the index case arose were estimated to be 1200 for contacts in the household, 160 in secondary schools, 60 in primary schools, 1.8 in universities/colleges, and 0 in nurseries. Between seven and 30 days the relative risks were lower; 150 in households, and between 0 and 13 in all other settings. Beyond 30 days, the relative risk in the household setting was 8 and lower than this in all other settings. The absolute risk of further cases in the month following the index case was calculated as 210 per 100,000 in household members, 7-10/10(5) in pupils at the same school, and 0.6/10(5) in students at the same university or college. The current policy in England and Wales to recommend chemoprophylaxis for household members may prevent half of the further cases in this setting. Raised awareness may have contributed to the lower case fatality rate among household contacts who developed meningococcal disease, but the number of co-primary cases observed should prompt urgent enquiries about current illness in household contacts of index cases. The relative risk of further cases in preschool groups was low and apparently unaffected by changes in chemoprophylactic policy. The relative risk in school settings was raised in the month following a case, but the absolute risk was still low. Further study to quantify the risk in university settings is needed.