As international travel continues to rise, the importance of proper vaccination against mosquito-borne diseases like yellow fever has come under scrutiny, particularly for young travelers. A recent clinical study published in The Lancet has revealed that infants may require a full dose of the yellow fever vaccine to ensure adequate protection.
Each year, numerous children venture into regions where the yellow fever virus is prevalent, prompting health authorities to reconsider vaccination protocols. The study, which was published on January 13, 2026, indicates that a low-dose vaccine (500 IU) does not provide the same degree of immune defense in infants as the standard dosage.
The randomized, double-blind trial involved infants aged 9 to 12 months who had not previously received a yellow fever vaccination. Researchers administered either the standard dose, consisting of over 13,000 IU, or the reduced dose to the participants. The main goal was to assess seroconversion rates 28 days after vaccination, defined as a four-fold or greater increase in antibody levels.
The findings were compelling: 99% of infants who received the standard dose achieved seroconversion, compared to 93% in the low-dose group. This represents a significant difference of 6.15 percentage points, underscoring the ineffectiveness of the lower dose in providing sufficient immunity for infants.
Both groups experienced similar safety outcomes, with 12 serious adverse events reported. Importantly, these incidents were found to be unrelated to the vaccine itself. The researchers concluded that the rationale for using fractional doses in adults does not extend to infants, suggesting that a higher dosage is essential for proper immunization.
In an accompanying commentary, Dr. Lance Turtle from the University of Liverpool noted that while fractional dosing has been employed during yellow fever outbreaks, it should not be used as a standard practice for regular vaccinations in infants.
According to Don Hackett, publisher of Vax-Before-Travel, the study highlights the necessity for infants to receive standard doses to ensure long-lasting protection against yellow fever, particularly for travel to endemic areas such as Brazil.
The Centers for Disease Control and Prevention (CDC) currently advises that children aged 9 months and older who are traveling to or residing in high-risk zones receive the YF-VAX yellow fever vaccine. However, infants under 9 months, pregnant women, and individuals with specific health concerns are advised against vaccination.
Travel vaccine specialists are available to provide guidance on the YF-VAX vaccine when departing from the United States to areas at risk of yellow fever in 2026.
