Background
Yellow Fever (YF) is a mosquito-borne viral disease endemic to tropical and subtropical regions in Africa and the Americas. Infection with yellow fever virus can result in subclinical to severe illness, characterized by fever, jaundice, and hemorrhage.
All currently used YF vaccines are live attenuated viral vaccines derived from the 17D strain. Nearly all studies have shown that one dose induces seroconversion in more than 98% of recipients, and protection is believed to be lifelong.
A fractional dose of the 17DD YF vaccine was effective at inducing seroconversion in participants who were seronegative at baseline. Titers remained above the threshold for seropositivity at 1 year after vaccination in nearly all participants who were seropositive at 1 month after vaccination.
After reviewing evidence, WHOꞌs Strategic Advisory Group of Experts (SAGE) on Immunization, determined that a fifth of a standard YF vaccine dose can provide full protection against the disease for at least 12 months and can be used to control outbreaks.
In a mass vaccination campaign in Democratic Republic of the Congo in 2016 the fractional dosing method was shown to be feasible and a promising approach to protect at-risk populations that would otherwise be left unprotected.
YF vaccine fractional dosing is not proposed for routine immunization, as there is not enough data available to show that lower doses confer life-long protection. Studies are ongoing to determine the long-term protection provided by fractionated doses.
Braziian 2025 Outbreak
Ministry of Health issues alert for increase in YF cases in São Paulo and three other states in Brazil.
On February 2nd, 2025 – the Brazilian Ministry of Health issued a warning about the increase in the transmission of yellow fever during the seasonal period of the disease, which runs from December to May, especially in São Paulo, as well as Minas Gerais, Roraima and Tocantins. The technical note, sent to the Health Departments, recommends the intensification of surveillance and immunization actions in risk areas.
The state of São Paulo will account for the majority of yellow fever cases in 2025. Therefore, the Ministry of Health has decided to increase the number of doses of the vaccine sent to the state government. The state will receive two million doses by the beginning of February, with an additional 800,000 doses. Of these, one million were already delivered in January.
The amount was defined at a meeting of the Public Health Emergency Operations Center for Dengue and other Arboviruses (COE Dengue), coordinated by the department, last
Guidelines for travelers
People planning to travel to areas where yellow fever is transmitted or to rural and forested regions should check their vaccination records. Those who have not yet been vaccinated or received a fractional dose in 2018 should seek out a health unit at least ten days before traveling to get immunized and avoid exposure to the virus without protection.
The same recommendation applies to: populations living in locations with evidence of viral circulation or in rural areas; riverside populations and those living around parks and conservation units; rural, agricultural, extractive and environmental workers, among others; individuals with sporadic exposure in risk areas (rural, wild); and travelers to affected areas, including workers and tourists/ecotourists.
Vaccination is the main tool for preventing yellow fever.
The vaccine is part of the basic NIP vaccination schedule for children aged 9 months to under 5 years, with a booster dose at 4 years of age, in addition to a single dose for the population aged 5 to 59 who have not yet been vaccinated.
Other recommendations
Booster dose: individuals who received the fractional dose of the yellow fever vaccine in 2018 and who will travel to areas with proven circulation of the virus should receive an additional dose in the standard presentation.
Zero dose: administered between 6 and 8 months of age, this dose should only be administered to children who reside in or will travel to areas with confirmed circulation of the virus.
Vaccination of the elderly: people aged 60 or over must undergo an individualized medical evaluation before vaccination, considering the risk of exposure to the virus and their health conditions.
In addition to vaccination, it is essential to adopt personal protective measures, such as wearing long-sleeved pants and shirts, closed shoes, and applying insect repellent to exposed areas of the body. Since yellow fever virus vectors are diurnal, these precautions must be maintained throughout the day.
REFERENCES
- Brazilian MOH News: https://www.gov.br/saude/pt-br/assuntos/noticias/2025/fevereiro/ministerio-da-saude-emite-alerta-para-o-aumento-de-casos-de-febre-amarela-em-sao-paulo-e-outros-tres-estados
- https://pmc.ncbi.nlm.nih.gov/articles/PMC7064153/ Casey RM,and cols. Immunogenicity of Fractional-Dose Vaccine during a Yellow Fever Outbreak – Final Report. N Engl J Med. 2019 Aug 1;381(5):444-454. doi: 10.1056/NEJMoa1710430. Epub 2018 Feb 14. PMID: 29443626; PMCID: PMC7064153.
- WHO: https://www.who.int/news-room/questions-and-answers/item/fractional-doses-of-the-yellow-fever-vaccine
- https://asm.org/articles/2021/august/yellow-fever-and-fractional-vaccine-doses Yellow Fever and Fractional Vaccine Doses – Andrea Prinzi, Ph.D., MPH, SM(ASCP) – ASM Articles, Aug. 31, 2021.
