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. However, some vaccine recipients develop mild systemic reactions, including myalgia and headache. Contraindications include true allergy to egg protein, immunodeficiency (congenital or acquired) and symptomatic HIV infection (Chapter 9). There is a theoretical risk of harm to the fetus if the vaccine is given during pregnancy and vaccination of nursing mothers should be avoided because of the risk for the transmission of 17D virus to and encephalitis in the breast-fed infant.These risks must be weighed against the risk to the mother of remaining unvaccinated and travelling to an area where exposure to YFV may occur. In general, unvaccinated pregnant or nursing women should be advised not to travel to such areas.

Hypersensitivity reactions are rare, particularly anaphylactic reactions. However, the vaccine is produced in embryonated chicken eggs and is contraindicated in persons with a history of serious oral egg intolerance or strong allergic reactions to egg-based products.

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 of persons more than 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

Booster: Currently every 10 years (if re-certification is needed)

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 travelling to country or area at risk of transmission is unavoidable.

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