Among outbreaks caused by a single serogroup (n = 66; excluding two O26/O103 co-infection outbreaks), the stx1-producing strain was the most predominant (35/66, 53 %) (Table 4); 19/66 (29 %) outbreaks were associated with both the stx1- and stx2-producing strains and 12/66 (18 %) outbreaks were associated with the stx2-producing strain. Here, STX2 is linked to coinfection.