EDITOR—The paper by Neal et al documenting risk factors for acquisition of Neisseria meningitidis among university students helps further our understanding of the aetiology of outbreaks of meningococcal disease at universities.1 Several outbreaks have now been linked to bars and nightclubs.2 3 By showing that social factors such as attendance at bars influence acquisition, the study helps substantiate evidence that social behaviour is important in determining outbreak occurrence.

However, the threefold rise in carriage rates in the first four days of term is unexpected. The initial carriage rate (6.9%), as Neal et al acknowledge, is surprisingly low. In 16–24 year olds, carriage rates in both outbreak and non-outbreak situations are usually 20% or higher.4 5 The sudden increase, in a cross sectional study, to rates that would be normal for this age group suggests potential confounding. Carriage has been clearly documented to vary with age and sex, 2 4 5 yet no comparison of the age and sex of the four groups is presented, nor are these variables included in …