Prevalence of de-O-acetylated serogroup C meningococci before the introduction of meningococcal serogroup C conjugate vaccines in the United Kingdom

Meningococcal serogroup C conjugate (MCC) vaccines have been introduced in the UK to combat the rise in serogroup C meningococcal disease. Serogroup C meningococci may occur naturally expressing either O-acetylated (Oac⁺) or de-O-acetylated (Oac^-) polysaccharide capsules. In a small study in the USA in the 1970s 15% of serogroup C meningococcal case isolates were reported to be Oac^- though the prevalence of these Oac^- isolates has not been recorded in the UK. This is of interest as the first MCC vaccines to be introduced are Oac⁺ and the potential impact of this on Oac^- serogroup C isolates is unclear. Serogroup C isolates submitted to the Public Health Laboratory Service Meningococcal Reference Unit in January 1998 (n=113) and January 1999 (n=162) were investigated by dot blotting using monoclonals specific for Oac⁺ and Oac^- serogroup C polysaccharides. This revealed 12% Oac^- isolates for both January 1998 and January 1999. The proportion of fatal cases was found to similar for both Oac^- and Oac⁺, 14 and 9% for 1998 and 5 and 3% for 1999, indicating that the pathogenic potential of these Oac^- isolates is similar to Oac⁺. The acetylation status of serogroup C isolates needs to be monitored throughout and after the introduction of MCC vaccines.