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Christopher Aguilar
Professor Galvez
Bio 1615
28 March 2015
Yellow Fever Vaccine Safety Summary
Introduction
Immunity provided by the Yellow Fever Vaccine (YFV) is long-lasting. In addition, serious
adverse reactions (SAR) are rare. SARs are defined as neurologic or viscerotropic syndromes,
and anaphylaxis. A review of electronic databases and articles referenced in the database
contents were reviewed to quantify the frequency of SAR events in at-risk populations, described
as children, pregnant women, HIV+ individuals, and the elderly. Of the 2,415 abstracts reviewed,
32 were determined to meet the SAR criteria required for inclusion in the report. No other types
of SARs were observed in the at-risk groups.
Background
Yellow fever (YF) is endemic in South America and Africa, primarily in the tropical
regions. It can often go undetected due to its similarity to other illnesses, such as malaria, viral
hepatitis, typhus, and hemorrhagic fevers such as Ebola. This is compounded by the tendency for
YF infections to be asymptomatic, with an estimated 15% of infections developing moderate or
severe jaundice (Thomas et. al, 359).
YFV SAR events are very rare. The neurologic and viscerotropic syndromes that occur
are often hard to distinguish - just as with an actual YF infection - from diseases such as
meningitis, malaria, or Ebola. When they do occur, the course of action tends to be intensive
care, as no effective medication for SARs exists. Despite the remote possibility of SAR, the
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World Health Organization recommends people living in countries at risk of YFV infection to be
vaccinated. The rarity of SAR and the estimated 40 years of immunity from the vaccines
supports the WHO’s stance on the YFV vaccine. The most common reaction to the YFV from
those who are unvaccinated is a mild to low-grade fever, headache, and body pain (Thomas et.
al, 360).
The intent of the report was to identify the frequency of SAR in vaccine recipients who
are deemed at risk by one of the following: being children, pregnant women, HIV +, having
cancer or otherwise having compromised immunity, taking immunosuppressive medication, and
being elderly.
Materials and Methods
The study was conducted by identifying systematic reviews of incidents that contained
neurotropic and viscerotropic disease, and anaphylaxis or hypersensitivity, as well as other lifethreatening events. The systematic reviews were searched for using various electronic databases,
such as Cochrane Library (this includes the Cochrane CENTRAL Register of Controlled Trials,
Cochrane Database of Systematic Reviews, and the UK National Health Service Database of
Abstracts of Reviews of Effects), MEDLINE, EMBASE, BIOSIS Previews, Global Health, CAB
Abstracts, and the Lilacs Database of Latin American and Caribbean literature.
The abstracts were reviewed independently by two reviewers to pinpoint those that met
the criteria for at-risk groups afflicted by SAR due to receiving the YFV. The results were
separated into two categories – active surveillance, well-designed studies using randomized
controlled trials (RCT), and passive surveillance, which did not use these methods and relied
solely on observation. Care is to be taken when reaching conclusions due to the small sample
size identified in the reviews.
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Results and Discussion
4091 records were identified originally. After removing duplicates, 2415 records
remained. This number dropped to 472 after additional screening. 32 studies were settled upon
when they were showed to contain sufficient data highlighting incidents of SARs on the target
population. These 32 studies consisted of 9 studies on children, 8 studies on pregnant women, 9
studies on HIV+ patients, 5 studies on elderly 60 or older, and one study on patients taking
immunosuppressants.
1866 infants were vaccinated in RCTs, and none showed any SAR in 4 studies using
active surveillance. In 5 studies using active surveillance of infants receiving the YFV, 5 reports
of infant transmission after the mother received the YFV. This is considering 35000 YFV had
been given in Lagos with no reports of SAR there (Thomas et. al, 364).
1381 pregnant women were observed via active surveillance after receiving the YFV, and
rates of illness similar to SAR above women who had not received YFV were not found. Passive
surveillance found statistically higher abortion rates, but no incidents of SARs in the pregnant
women.
Studies of hundreds of patients with compromised immune systems due to HIV or
immunosuppressants, either with active or passive surveillance, failed to yield any concrete
instances of SAR when the YFV was involved.
Studies of a large sample of elderly patients, approximately 1.6 million over several
years, yielded a small amount of SAR incidents (34 instances) in which only the YFV was
involved, and a small number of SAR incidents (43 incidents) in which YFV was administered
along with other vaccines.
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Overall, the results point to an extremely small incidence rate of SAR in all at-risk
populations who received the YFV, with the elderly being most at-risk of experiencing SAR.
Conclusion
The data collected from the reviews of the studies was insufficient to change any current
position on the safety of YFV in at-risk populations, despite the small sample size in some of the
individual groups reviewed. The incidents of SAR in at-risk populations were negligible in the
studies reviewed. Given the information derived from the reviews, there are no recommendations
to update or modify any current vaccine precautions pertaining to the YFV.
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Bibliography
Thomas, Roger E., Lorenzetti, Diane L., Spragins, Wendy, Jackson, Dave, Williamson, Tyler.
"The Safety of Yellow Fever Vaccine 17D or 17DD in Children, Pregnant Women, HIV+
Individuals, and Older Persons: Systematic Review." American Journal of Tropical
Medicine and Hygiene (2012): 359-372. Web.