Table 2. The clustering of cases in several invasive diseases in England and Wales.

Disease	Variance to mean	Confidence interval
Meningococcal septicemia	49.44	36.56–53.97
Meningococcal meningitis	13.25	10.00–14.83
Hib meningitis	6.61	4.87–7.27
Pneumococcal meningitis	4.91	3.87–5.48

Clustering of cases, which is measured as the ratio of the variance to the mean of the annual incidence and based on the number of notified cases of the disease per year per health region, and 90% bootstrap confidence intervals are given. The data spanned the years 1982–1999 (meningococcal meningitis), 1989–1999 (meningococcal septicemia), 1982–1992 (H. influenzae b meningitis), and 1989–2002 (pneumococcal meningitis). Data source, Health Protection Agency Communicable Disease Surveillance Centre.