A cohort study of the duration of immunity to yellow fever after 4 years of primary vaccination in children and adults.

BACKGROUND

Lifelong protective immunity from a single dose of yellow fever (YF) vaccine remains controversial. We evaluated variations in YF neutralizing antibody titres over 4 years after primary vaccination.

METHODS

A cohort study was conducted in three municipalities in northeastern Brazil. The 17DD vaccine was administered between 2016 and 2018. Blood samples were collected to measure YF neutralizing antibodies before, 30-45 d, 1 y and 4 y after vaccination. Generalized linear mixed models with a Tweedie distribution were constructed for children and adults separately to estimate variation in antibody titres over the 4-y period.

RESULTS

Of the 4761 individuals, 2753 were children between 9 months and 4 y of age and 2008 adults ages 18-50 y. Four years after vaccination, seropositivity rates were 67.9% and 72.1% in children 9-23 months and 2-4 y, respectively, and 91.2% and 94.4% in adults 18-35 and 36-50 y, respectively. The ratio of means for neutralizing antibody titres 4 y after vaccination decreased to about one-third of that observed at 30-45 d after vaccination.

CONCLUSIONS

The evidence of waning immunity after a single dose of YF vaccine appears to support revaccination if primary vaccination targets infants and toddlers in endemic countries.