International travel and health

Yellow fever

Vaccine

The 17D vaccine, which is based on a live, attenuated viral strain, is the only commercially available yellow fever vaccine. It is given as a single subcutaneous (or intramuscular) injection. Yellow fever vaccine is highly effective (approaching 100%). All individuals aged 9 months or older and living in countries or areas at risk should receive yellow fever vaccine.

Precautions and contraindications

With the exception of very rare cases of vaccine-associated neurotropic and viscerotropic disease (see below), the 17D vaccine is generally considered to be safe. Contraindications include severe hypersensitivity to egg antigens and severe immunodeficiency. Conditions and treatments considered to be severely immunocompromising include: primary immunodeficiencies, thymus disorder, symptomatic HIV infection or CD4 T-cell values <200 per mm3, malignant neoplasm treated with chemotherapy, recent haematopoietic stem cell transplantation, drugs with known immunosuppressive or immunomodulatory properties (e.g. high-dose systemic corticosteroids, alkylating drugs, antimetabolites, TNF-α inhibitors, IL-1 blocking agent, or other monoclonal antibodies targeting immune cells), and current or recent radiation therapies targeting immune cells.

Noting that yellow fever vaccine is a live vaccine, a riskbenefit assessment should be undertaken for all pregnant and lactating women. In areas where YF is endemic, or during outbreaks, the benefits of YF vaccination are likely to far outweigh the risk of potential transmission of vaccine virus to the fetus or infant. Pregnant women and nursing mothers should be counselled on the potential benefits and risks of vaccination so that they may make an informed decision about vaccination. Lactating women should be advised that the benefits of breastfeeding far outweigh alternatives. Vaccination is recommended, if indicated, for pregnant or breastfeeding women travelling to endemic areas when such travel cannot be avoided or postponed.

The YF vaccine is contraindicated in infants under 6 months of age and is not recommended for those aged 6–8 months, except during epidemics when the risk of YFV transmission may be very high.

Viscerotropic disease: Vaccine-associated viscerotropic disease is a recently described adverse event that on very rare occasions has occurred after the first immunization with the yellow fever 17D vaccine. Onset is within 10 days of vaccination and the pathological process is characterized by severe multi-organ failure and an overall case–fatality rate in excess of 60%. Known risk factors include a history of thymus disease (e.g. thymoma or thymectomy) and age ≥60 years. In the USA the risk of contracting viscerotropic disease after YF vaccination for persons over 70 years of age is estimated to be 2.4 cases/100 000 vaccine doses.

Neurotropic disease: Increased incidence of vaccine-associated neurotropic disease (e.g. meningoencephalitis, acute disseminated encephalomyelitis and Guillain–Barré syndrome) has been reported in infants under 6 months of age and in vaccine recipients aged 60 years and older. The reported rate of vaccine-associated neurotropic disease in travellers from the United States and Europe ranges between 0.13 and 0.8 per 100 000 doses.

Yellow fever vaccination is required for travellers to certain countries and is recommended for all travellers to countries or areas with risk of yellow fever transmission (see Country list and Annex 1).

While yellow fever vaccination should be encouraged as a key prevention strategy, it is important to screen travel itineraries and carefully evaluate the potential risk of systemic illness after yellow fever vaccination. Great care should be exercised not to prescribe yellow fever vaccination to individuals who are not at risk of exposure to infection, based on an accurate assessment of the travel itinerary. Although vaccination is generally not recommended for travellers going to areas where the risk of exposure is low, any risk (e.g. as a result of prolonged travel or heavy exposure to mosquito bites) should be weighed against individual risk factors for vaccineassociated adverse events (e.g. altered immune status).

Type of vaccine: Live, attenuated

Number of doses: One dose of 0.5 ml

Boosters: A single dose of YF vaccine is sufficient to confer sustained lifelong protective immunity against YF disease; a booster dose is not necessary for protection but may still be required by some countries. Adjustments of the provisions for the duration of validity of certificates under the IHR are ongoing.

Contraindications: Infants aged less than 6 months; history of severe allergy to egg or to any of the vaccine components, or hypersensitivity to a previous dose of the vaccine; thymoma or history of thymectomy, immunodeficiency from medication, disease or symptomatic HIV infection

Adverse reactions: Rarely, neurological (encephalitis) or multi-organ failure resembling wild-type yellow fever.

Before departure: International certificate of vaccination becomes valid 10 days after vaccination.

Recommended for: All travellers to countries and areas with risk of yellow fever transmission and when required by countries.

Special precautions:Not recommended for infants aged 68 months, except during epidemics when the risk of YF virus transmission may be very high. The risks and benefits of vaccination in this age group should be carefully considered before vaccination. The vaccine should be avoided during pregnancy or breastfeeding. However, pregnant or nursing women may be vaccinated during epidemics or if travel to a country or area at risk of transmission is unavoidable.

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