Fig. 1 summarises the serotype specific seropositivity rates (titres >10) during follow up across the two study groups. The GMTs for the four dengue serotypes decreased during follow up. The decrease was most pronounced within the first year post-vaccination, but remained 2- to 4-fold higher than at baseline over the five years of follow-up for all serotypes, age groups and study groups. There was little difference in GMTs for each serotype between the two groups for the duration of follow up (Supplementary Fig. 1). Natural exposure to wild-type dengue, as determined by a 4-fold increase in PRNT50 titre (and with a resultant titre ⩾40), occurred in approximately 10% of participants each year (35 out of 77 participants in the TDV–TDV–TDV group had evidence of at least one episode of exposure to wild-type dengue). Participants with no evidence of natural exposure to wild-type dengue in the TDV–TDV–TDV group had generally lower seropositivity rates (data not shown), particularly over time, and lower GMT levels for all dengue serotypes compared with all participants throughout follow up (Fig. 2), suggesting that natural exposure to wild-type dengue boosted immunogenicity of all serotypes. The same trends regarding antibody persistence and natural exposure to wild-type dengue were observed for participants in the TyVi–TDV–TDV group (data not shown). Higher GMTs were observed for all time points in participants who were flavivirus positive at baseline (Fig. 3).