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- - - YELLOW FEVER VACCINATION

Saturday, 25th of May 2013

• YELLOW FEVER VACCINATION

A report was presented from the SAGE working group on yellow fever (YF) vaccines; the group was tasked with reviewing the evidence and making recommendations to SAGE with a view to updating the 2003 WHO position paper on the use of YF vaccines. An extensive background paper was provided; it was in particular informed by 2 systematic reviews on, respectively, whether there is a need for booster doses of YF vaccine every 10 years after primary vaccination, and on the risk of serious adverse effects following immunization in the elderly.

Based on currently available surveillance data, SAGE concluded that vaccine failures are extremely rare and do not cluster as time increases after immunization. A single dose of YF vaccine is sufficient to confer sustained immunity and life-long protection against yellow fever disease and a booster dose of YF vaccine is not needed. Surveillance in endemic countries and clinical studies may possibly identify specific risk groups (such  as infants or HIV-infected patients) that could benefit from a second primary or booster dose. SAGE requested WHO to revisit the IHR provisions relating to the period of validity for international certificates for vaccination against YF.

Regarding the use of YF vaccine in people over 60 years of age, SAGE noted that while the risk of YF vaccine-associated viscerotropic disease in persons ≥60 years of age is higher than in younger groups, the overall risk remains low. Vaccination should be recommended based on a careful risk–benefit assessment comparing the risk of acquiring yellow fever disease versus the risk of a potential serious adverse event following immunization for persons ≥60 years of age who have not been previously vaccinated and for whom the vaccine is recommended.

Further research is needed to better quantify the risk for vaccine recipients who are ≥60 years of age and who reside in or travel to a yellow fever endemic area.YF vaccine is not recommended for individuals who are severely immunocompromised for a range of clinically recognized reasons. YF vaccination may be offered to asymptomatic HIV-infected persons with CD4+ counts ≥200 cells/mm³ who require vaccination. There is limited clinical study data on safety and immunogenicity of YF vaccine when used in HIV-infected children. However, available data are reassuring on safety, including from a secondary analysis of safety in a mass vaccination campaign in Brazil, although the vaccine may be less immunogenic in these children. In addition, YF vaccine has been used in routine immunization programmes where HIV-infected children have been vaccinated with no safety concerns noted. SAGE therefore recommended that the vaccine may be administered to all clinically well children through immunization programmes, and that HIV testing is not a pre-requisite for vaccination in this setting.

In situations where the risk of YF disease is high and mass vaccination campaigns are undertaken, limited data has not so far indicated safety concerns for HIV-positive adults and children who are immunized in this context. SAGE therefore recommends that for mass immunization campaigns there is no requirement to establish HIV status as a prerequisite for vaccination.

Additional data on safety and immunogenicity of YF vaccine, including persistence of immunity in HIV-positive adults and children should be obtained. There are limited data on the use of YF vaccine in pregnant and lactating women. Current data do not suggest a risk of viscerotropic or neurologic disease in mothers or their fetus/newborn after immunization and there is no evidence of congenital abnormalities due to YF vaccine. There were 3 cases of virus transmission shown in lactating mothers.

Vaccination is nevertheless recommended if indicated for pregnant or breastfeeding women travelling to endemic areas when travel cannot be avoided or postponed. Both pregnant women and nursing mothers at high risk of yellow fever disease should thus be counseled regarding the benefits and potential risks of vaccination so that they can make an informed decision about vaccination. For lactating women, the benefits of breastfeeding infants far outweigh those of other nutritional alternatives.

Limited data are currently available on the safety and immunogenicity when YF vaccine is simultaneously administered with other vaccines. Although several studies have indicated that the YF and measles vaccines can be simultaneously administered without any effects on safety and immunogenicity, a single study of simultaneous administration of YF and measles, mumps and rubella (MMR) vaccines in infants suggest that immunogenicity may be compromised for both YF vaccine and the rubella and mumps components of MMR vaccine. Separating MMR and YF vaccine administration by 30days mitigated the effect. To date, there is insufficient evidence to change current recommendations and SAGE recommended that additional studies should be undertaken on the simultaneous administration of YF and other vaccines, to further inform immunization programmes.

The control strategy for YF should include sound epidemiologic surveillance, and delivery of YF vaccine through a complementary and optimized combination of routine immunization and mass preventive campaigns. Reactive campaigns should be conducted in response to yellow fever outbreaks if there is inadequate vaccination coverage within the population.

SAGE recommended that all countries with areas at risk should set time-defined objectives for the introduction of YF vaccine into their immunization programmes and to establish regional plans for controlling yellow fever.