A one-fifth dose of the 17D-YFV vaccine against yellow fever induced a protective immune response that lasted for 10 years, according to study results published in Annals of Internal Medicine.

Investigators performed a 10-year follow-up study (Dutch Trial Register: NTR7094) on a subgroup from a randomized controlled non-inferiority trial (Dutch Trial Register: ISRCTN46326316) to evaluate the long-term immunogenicity of the fractional dose vaccination.

The study evaluated blood samples from 75 of the original 155 participants, who had been randomly assigned to receive either the standard subcutaneous dose of the vaccination or the fractional intradermal dose. Of these patients, 40 individuals received the 0.1-ml fractional dose and 35 received the standard 0.5-ml dose. Virus-neutralizing antibody responses were measured by a plaque reduction neutralization test. Of the 40 individuals who received the fractional dose, 98% (95% CI, 89%-100%) had protective levels of neutralizing antibodies at the 10-year follow up, as did 97% of the individuals who received the standard dose. 

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Researchers noted that in patients in endemic regions receiving the fractional dose it was unclear how chronic parasitic infections or malnutrition may affect long-term immunity. Also, they cautioned that there is a considerable range of potency among yellow fever vaccine batches; therefore the conclusions drawn from fractional dose studies must consider batch potency. 

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With these limitations in consideration, the researchers concluded that “these findings reinforce the policy of using fractional doses in preventive mass vaccination campaigns before an impending outbreak and show that a booster dose of 17D-YFV vaccine is not necessary after receipt of a fractional dose.”


Roukens AHE, van Halem K, de Visser AW, Visser LG. Long-term protection after fractional-dose yellow fever vaccination: follow-up study of a randomized, controlled, noninferiority trial. Ann Intern Med. 2018; 169: 761-765.