Yellow fever vaccine and HIV infection
WHO recommends that all people aged ≥9 months living or travelling in areas where there is a risk of yellow fever transmission should be vaccinated. However, the vaccine is contraindicated for people who are severely immunocompromised.6 The benefits of mass vaccination campaigns for yellow fever are recognized in endemic countries, and millions of individuals are vaccinated against the disease every year in countries where the prevalence of HIV is 1–5% among those aged 15–49 years. In many places access to laboratory testing and other resources for diagnosing and treating HIV infection is poor, and many people with undiagnosed advanced HIV infection are likely to have received the vaccine.
At its June 2008 session, GACVS recommended that in light of the significant number of doses of yellow fever vaccine being delivered (and planned to be delivered) in preventive vaccination campaigns in endemic countries, some of which have a significant prevalence of HIV infection, appropriate follow-up studies after vaccine use should be conducted to improve the data on the safety and immunogenicity of this vaccine in individuals infected with HIV.7 GACVS has reviewed the latest data from the limited published literature and the preliminary reports of experience monitoring vaccination campaigns in Africa and Latin America.
Published studies on the safety and immunogenicity of yellow fever vaccines in HIV-positive people are limited to small studies and case reports, mainly of travellers with CD4 >200 cells/mm3. With the exception of 1 case of fatal meningoencephalitis, these studies did not detect any other serious adverse events following immunization (AEFI) among HIV-positive individuals. However, little evidence has accumulated about the safety of this vaccine in people with advanced HIV infection. Data about the immune response to the vaccine are scarce but show consistent immunogenicity in HIV-positive people with CD4 counts >200 cells/mm3.
In West and Central Africa, between 2007 and 2010, 10 countries undertook vaccination campaigns against yellow fever, during which about 50 million people were vaccinated. In these countries, surveillance efforts have been implemented in collaboration with national health authorities and local expert committees. Analyses of the safety data are continuing in 7 countries, but so far around 194 serious AEFI have been reported, and more than three quarters of patients have been tested for HIV. Only a few individual cases of serious AEFI have occurred in HIV-positive individuals. Similar findings have been reported from vaccination campaigns in Latin America.
In summary, monitoring vaccination campaigns in countries where the prevalence of HIV is about 1–5% has identified only a few HIV-positive individuals among those with any serious AEFI; no clear risk has been identified that precludes the use of yellow fever vaccine in people infected with HIV. However, the sensitivity of these studies to detect serious AEFI has not been established. In addition, GACVS is awaiting data about the completeness of case investigation, the classification of serious AEFI and the HIV status of those cases.
No changes have been suggested by GACVS to WHO’s recommendation that individuals known to be severely immunocompromised should not receive yellow fever vaccine; the available data do not identify a significant problem with mass vaccination in populations where a moderate proportion of individuals are HIV-positive. However, GACVS strongly recommends that additional data on safety and immunogenicity should be obtained on the effect of vaccination in HIV-positive individuals, and especially in those with advanced HIV infection. This could be accomplished by strengthening systems for detecting and investigating serious AEFI during vaccination campaigns. Also, additional clinical studies of yellow fever vaccines administered to HIV-positive individuals should be conducted.
6See No. 40, 2003, pp. 349–359.
7See No. 32, 2008, pp. 287–292.