Further data on the need for booster doses of yellow fever vaccine (YFV)

INTRODUCTION: Available scientific evidence to recommend or to advise against booster doses of YFV is inconclusive. A study by the Fiocruz Collaborative Group for the Study of Yellow Fever Vaccines estimated the seropositivity rate and geometric mean titer (GMT) of adults with varying times of vaccination was aimed to provide elements to revise the need and the timing of revaccination.

METHODS: Adults from the cities of Rio de Janeiro and Alfenas located in non-endemic areas in the Southeast of Brazil, who had one dose of YFV, were tested for YF neutralizing antibodies and dengue IgG. Time (in years) since vaccination was based on immunization cards and other reliable records.

RESULTS: From 2011 to 2012 we recruited 691 subjects (73% males), aged 18-83 years. Time since vaccination ranged from 30 days to 18 years. Seropositivity rates (95%C.I.) and GMT (International Units/mL; 95%C.I.) decreased with time since vaccination: 93% (88%-96%), 8.8 (7.0-10.9) for newly vaccinated; 94% (88-97) , 3.0 (2.5-3.6) after 1-4 years; 83% (74-90), 2.2 (1.7-2.8) after 5-9 years; 76% (68-83), 1.7 (1.4-2.0) after 10-11 years; and 85% (80-90), 2.1 (1.7-2.5) after 12 years or more. YF seropositivity rates were not affected by previous dengue infection.

CONCLUSIONS: Serological correlates of protection for yellow fever are unknown. Seronegativity may indicate primary immunization failure, and waning immunity may lead to levels below the protection threshold. Immunogenicity of YFV under routine conditions of immunization services is likely to be lower than in controlled studies. Moreover, infants and toddlers, who comprise the main target group in YF endemic regions, and populations with high HIV infection rates, respond to YFV with lower antibody levels. In those settings one booster dose might be anticipated to ensure longer protection for all vaccinees.