Duration of immunity in recipients of two doses of 17DD yellow fever vaccine.

Antibody levels following yellow fever vaccination (YFV) decrease considerably with time, leaving a substantial proportion of single-dose recipients seronegative after 10 years. Little is known about the persistence of immunity after two or more doses of the vaccine. This study was designed to verify the current immune status of adults who had received two or more doses of YFV. A cross-sectional study assessed the immune status for yellow fever according to time since the latest YFV: second dose at 30-45 days (reference group), 1-5 years, 6-9 years and 10 years or more; and three or more doses. Volunteers had their vaccination cards checked and were interviewed before having a blood sample drawn for titration of neutralizing YF antibodies, and being tested for dengue IgG. The proportion seropositive (titer ≥ 1:50) and the geometric mean titers (GMT) were compared among the subgroups of time since the latest YFV. Participants (n = 323) were predominantly female (80.5%) and had a median age of 34 years (range 18-76). In the reference group (n = 99), 69% were seropositive before the second dose. After revaccination, the proportions seropositive (95% C.I.) were 100% (96-100%) in the reference subgroup, 90% (83-95%) in the 1-5 year subgroup (n = 109), 86% (77-92%) in the 6-9 year subgroup (n = 92) and 86% (57-98%) in the 10+ years subgroup (n = 14). Only 9 participants with more than 2 previous doses of YFV were eligible for the study, and 8 of them were seropositive. Sociodemographic variables, age at vaccination and previous dengue infection did not confound the association of seropositivity to time since the last YFV. The results support the need of booster doses of the YFV to maintain antibody levels consistent with protection, and indicate that a small proportion of individuals may need more than two doses, provided that priority is given to primovaccination of all susceptibles.