Yellow Fever Vaccine: Adverse Events Appear Rare but Real

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mosquitoA highly effective live attenuated vaccine, developed more than 60 years ago from the 17D strain of yellow fever (YF) virus, has been a key tool in controlling this disease. The vaccine is given as part of national childhood immunization programs in some endemic areas, and many countries require travelers to provide proof of YF vaccination on entry. Three recent papers report adverse events following YF vaccination.

Researchers in the U.S. report 4 patients (63 and older) with multiple organ-system failure after vaccination with 17D-204 YF. Three died. Researchers found vaccine-related YF virus in the plasma and cerebrospinal fluid of 1 patient and YF antigen in the liver tissue of another. Convalescent sera (available for 2 patients) showed unusually high levels of antibodies.

Investigators in Brazil describe the deaths of 2 young people (aged 5 and 22 years) after YF vaccination. The 5-year-old had a history of chronic diarrhea and recurrent bronchitis. Clinicalillness in both cases resembled YF. In both patients, researchers isolated virus identical to the vaccine and found YF antigen in the liver tissue, which showed pathologic changes similar to those seen in wild-type YF. Approximately 2 million people received vaccinations from the same lots. During the decade 1990-2000, about 85 million doses of YF vaccine (17 DD vaccine) were given in Brazil because of an upsurge of YF in endemic areas and threats of YF in urban areas.

A report from Australia describes the death of a 56-year-old man 11 days after YF vaccination. Vaccine-strain virus was isolated from multiple serum samples and from the heart, liver, and muscle at autopsy. Histopathologic changes in the liver were characteristic of YF.

Comment: These reports suggest that yellow fever vaccine can, in rare instances, cause illness resembling infection with wild-type YF virus. Older age appears to be one but not the only risk factor. YF remains a serious infection, with a case-fatality rate of 20 percent to 50 percent. Outbreaks of YF have increased in recent years, and a YF-competent vector (Aedes aegypti) now infests most countries of the Americas, including urban areas. The YF vaccine has been remarkably safe and effective, and it should continue to be given to persons at risk of exposure to this disease. Outside endemic areas, the vaccine should be given only to persons traveling to endemic zones or areas reporting YF activity, and prospective recipients should be advised of potential adverse effects. These reports point to a need for enhanced surveillance for adverse events related to YF vaccination, detailed analysis of the vaccine for mutations, and study of host response to the virus. At the time of publication, the CDC report of these cases (MMWR Morb Mortal Wkly Rep 2001; 50:643) was available free at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5030a3.htm

 source - JournalWatch