Meningococci are gram-negative diplococci. They are divided into (at present) 12 capsular groups according to characteristics of the polysaccharide capsule. These groups are A, B, C, E, H, I, K, L, W, X, Y, and Z.

Historically, B, C, W and Y were the most common in the UK. However, due to vaccination programmes, cases of Men C have reduced dramatically.

Serogroup B meningococcus has been the most common cause of bacterial meningitis and septicaemia in recent years due to no licensed vaccination. In 2015, a Men B vaccine was introduced to the UK vaccination schedule. Cases of Men B have reduced by 75% in groups eligible for vaccination since then[33].

Since 2009, capsular group W infections have been forming an increasing proportion of cases[24].

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