17DD Yellow Fever Revaccination and Heightened Long-Term Immunity in Populations of Disease-Endemic Areas, Brazil

We evaluated the duration of neutralizing antibodies and the status of 17DD vaccine–specific T- and B-cell memory following primary and revaccination regimens for yellow fever (YF) in Brazil. We observed progressive decline of plaque-reduction neutralization test (PRNT) seropositivity and of the levels of effector memory CD4+ and CD8+ T cells, as well as interferon-γ+CD8+ T cells, 10 years after primary vaccination. Revaccination restored PRNT seropositivity as well as the levels of effector memory CD4+, CD8+, and interferon-γ+CD8+ T cells. Moreover, secondary or multiple vaccinations guarantee long-term persistence of PRNT positivity and cell-mediated memory 10 years after booster vaccination. These findings support the relevance of booster doses to heighten the 17DD-YF–specific immune response to guarantee the long-term persistence of memory components. Secondary or multiple vaccinations improved the correlates of protection triggered by 17DD-YF primary vaccination, indicating that booster regimens are needed to achieve efficient immunity in areas with high risk for virus transmission.


Additional Methods
We collected a total of 421 blood samples from 326 healthy men and women, 18-77 years of age. We categorized participants into 3 study arms: primary vaccination (reference groups), secondary vaccination, and multiple vaccination, on the basis of their status of having had a single dose of 17DD-YF vaccine, secondary (booster) dose, or multiple (>2) vaccination.
We sub-grouped participants according to distinct time-point before or after vaccination. The primary vaccination arm comprises 233 samples collected from 183 healthy adults of both sexes.
The secondary vaccination arm comprises 183 samples collected from 138 healthy adults of both sexes. The multiple vaccination arm comprises 5 samples collected from 5 healthy adults of both sexes.
For both the memory and biomarker analyses, data were reported as 17DD-YF Ag/CC Index. We performed first intra-arms analyses by paired T-test to compare the memory-related phenotypic features observed before vaccination with paired samples collected early after vaccination NV(d0) vs. PV(d30-45) and #PV(y≥10) vs. RV(d30-45). Additionally, we conducted intergroup analysis by ANOVA adjusted to multiple comparisons to compare the memory-related phenotypic features observed among distinct time-points: PV(d30-45) vs.

RV(y≥10).
In the comparative analyses between distinct time-points we paid special attention to the set of 3 phenotypic and functional biomarkers considered relevant universal attributes to monitor 17DD-YF specific memory: EMCD4, EMCD8, and IFNCD8. Particular attention was given to We assessed resultant memory by plaque-reducing neutralization test (PRNT) and EMCD8 measurement before and after primary and secondary 17DD-YF vaccination at participant level to calculate the overall proportion of participants for whom test results showed EMCD8 or PRNT levels above the cutoff threshold, i.e., PRNT positivity at serum dilution >1:50 and EMCD8 index above the global median value (17DD-YF Ag/CC Index >1.13). Then, we performed χ 2 tests to determine the resultant memory for each subgroup in the primary vaccination and secondary vaccination study arms. We noted significant differences at p<0.05.
We assessed immune response before and after primary, secondary, or multiple 17DD- Thereafter, additional intersection diagram was constructed to select the common biomarkers observed in PV(d30-45)RV(d30-45). The attributes EMCD4, EMCD8 and IFNCD8 were then underscored as universal memory-related biomarkers. These attributes were tagged in bold underline format and employed for follow-up analysis overtime after primary, secondary, or multiple 17DD-YF vaccination.
The results are expressed as reverse of serum dilution and seropositivity rates determined considering the serum dilution higher 1:50 as the cutoff criterion for PRNT positivity. Participant subgroups indicate number of days or years since vaccination (in parentheses; d0 for those never vaccinated).