Until 2001 it was believed that the vaccine against yellow fever (FG) was extremely safe. This belief was changed by the discovery of a severe complication observed, albeit rarely, in some people vaccinated for the first time.
viscerotropic disease is associated with yellow fever vaccine (YEL-AVD, Yellow Fever Vaccine-Associated Disease Viscerotropic), similar to the disease caused from wild-type virus and burdened by a fatality of 60%. This complication is determined by the 17D vaccine virus, which is affected in subjects devoid of mutations able to determine their virulence, so that the research is directed towards any risk factors present in vaccinated individuals.
two have been identified: the presence of a disease that involves the thymus (eg. Myasthenia gravis, or a previous thymectomy) and age over 60 years (probably due to involution of the thymus that manifests itself in old age). The incidence of the disease viscerotropic is estimated at 1 in 200,000 to 400,000 vaccinated, but if we consider only the sixties, this rises to 1 in 50,000.
These data would suggest caution in recommending a particular vaccination FG, since only the real people at risk should be vaccinated. In my previous post I explained the situation of yellow fever in Africa, pointing to a different epidemiological situation in eastern Africa than the western portion of the continent. In particular, the two nations, half of an intense flow of Western tourists, they can create problems in the counseling prevaccinale long as there have been no cases of yellow fever, although classified as risk areas: the Kenya and Tanzania.
The International Health Regulations considers Kenya and Tanzania, countries at risk of virus transmission FG, as the primary vector and host (non-human primates) are present. The WHO and CDC recommend vaccination FG to all visitors on the grounds that the transmission of the virus may occur, so all travelers should be protected. As a result of that position, many Western travelers are vaccinated against FG.
Ron Behrens An article published in 2008 in the Journal of Travel Medicine about his Kenya, the title is very explicit: "Yellow fever recommendations for tourists to Kenya: a flawed risk assessment? "or" anti-yellow fever vaccination for travelers in Kenya: a distortion in the risk assessment? ".
Author considerations can be summarized as follows:
- before 1992 and since 1997 have not been reports of FG in Kenya. An outbreak occurred near Baringo between 1992 and 1993 produced a total of 72 cases;
- strengthening of the surveillance in 2004 did not identify the presence of virus circulation. Although the current surveillance of arbovirus infections has proved effective in identifying outbreaks like that which occurred recently, Rift Valley Fever and therefore should also immediately identify any cases of FG;
- although Kenya has introduced universal vaccination against FG in children after the epidemic of 1992, it is difficult Vaccine coverage has exceeded 50%. Therefore, the absence of cases is not the result of high vaccination coverage so as to prevent the circulation of the virus in the population;
- recommendations for travelers should be balanced taking into account the recent reports of serious adverse events associated with vaccination, especially in elderly patients (> 60 years) vaccinated for the first time;
- Kenya is visited each year by 1.54 million people, out of 3 Britons who travel to Kenya, one is aged over 55 years. Based on these data, the current vaccination policy of the GF is capable of producing 10 to 13 serious adverse events or fatalities each year. This incidence was much higher than the risk of disease is calculated on a historical basis, is the potential risk of a possible epidemic in Kenya will involve a traveler.
In response to the article by Ron Behrens, the Journal of Travel Medicine has published a letter from Thomas Monath and others, in which some facts are emphasized:
- Kenya is bordered by Uganda, Sudan and Ethiopia, where the FG is endemic, with recurrent outbreaks;
- Rift Valley Fever virus causes disease in cattle, where monitoring is easier, while the animal reservoir of yellow fever (non-human primates) is not subject to surveillance in Kenya
- the monitoring of FG in the human population in Kenya is passive, so that the less severe forms or sporadic cases could not be detected;
- the ecological situation in which FG is the transmission of the virus has not changed over time.
For these reasons Monath et al. believe that the traveler in Kenya is to be considered at risk, especially if prolonged outdoor activities (resulting in exposure to vector Aedes) in the northern and western part of Kenya.
In its reply to the letter of Monath et al., Behrens considers that the present position of the WHO does not seem to take into account the peculiarities of the individual traveler and urges (or, shall I say, challenge) its partners to produce an estimate on where (and when) the risk of yellow fever in Kenya exceeds the risk of serious adverse events post-vaccination.
In my view, similar considerations for or against a particularly prudent approach towards vaccination, could be made in Tanzania, where there are no reported human cases for decades.
What is the risk of yellow fever in unvaccinated travelers?
We must consider two situations: the epidemic and the ongoing inter-epidemic period (silent, with no notification of cases of disease but with possible viral circulation).
The estimated risk for a stay of two weeks in an area with epidemic act is 1 in 267 (1 death on 1333).
A two-week stay in an area with possible transmission of the virus but without ongoing epidemic poses a risk of 1 in 1000 travelers per month (1 death per month in 5000).
So, for a typical stay of two weeks, the figure is 1 in 2000 travelers (1 death in 10000). You can compare these data with the incidence of disease in vaccinated viscerotropic.
Behrens are right or they are right Monath and colleagues?
Wanting groped to answer this question, first we must not forget that the traveler is able to reduce the risk of yellow fever simply by applying, with due care, the measures antivettoriali. We therefore need to spend some 'time For information on these instructions on how to be applied and which products to use. But it's worth it, because - in addition to being essential to combat malaria - such precautions are necessary to guard against diseases such as dengue fever, to which there are currently no other means of prevention.
Second, we must carefully assess the risk based on the itinerary, length of stay in endemic areas considered and the type of activities that increase the risk (eg. Prolonged stay outdoors, especially in forest areas, or villages that are located on the edge of the jungle).
Last but not least, the age and health of the traveler assume paramount importance in our evaluation.
The decision is taken with the traveler, informing them on risks of the vaccine and the risks of non-immunization. In situations where vaccination FG is required under the International Health Regulations there is no choice and a possible exemption for medical reasons can only be true if there is a contraindication to vaccination.
A budget risks and benefits is very simple (and it is clearly in favor of vaccination) when the destination of the trip is an area where the virus circulation is ongoing, as large areas of South America or Africa, while it may be problematic for countries such as Kenya and Tanzania, for the reasons set out just now. And here comes our ability to provide effective counseling in order to reach a joint decision and a conscious choice by the patient.
Personally (just my opinion but it is an operator on the field) I feel more in tune with the position of Ron Behrens.
to write this post, I used the following literature sources:
Ron Behrens. Yellow fever recommendations for tourists to Kenya: a flawed risk assessment? Journal of Travel Medicine 2008;5:285-286.
DOI: 10.1111/j.1708-8305.2008.00225.x
Thomas P. Monath, Mark Gershman, David R. Hill, Nina Marano, J. Erin Staples, Annelies Wilder-Smith. Yellow Fever Recommendations for Tourists to Kenya: A Flawed Risk Assessment. Journal of Travel Medicine 2009;16:146.
DOI: 10.1111/j.1708-8305.2008.00295_1.x
Ron Behrens. Reply. Journal of Travel Medicine 2009;16:146-147.
DOI: 10.1111/j.1708-8305.2008.00295_2.x
TP Monath, MS Cetro, DE Teuwen. Yellow fever vaccine. In: S. Plotkin, W. Orenstein, P. Offit. Vaccines. Fifth Edition. Saunders Elsevier 2008
ISBN 978-1-4160-3611-1.
Monath TP. Dengue and Yellow Fever-Challenges for the Development and Use of Vaccines. NEJM 2007, 357:2222-25.
DOI: 10.1056/NEJMp0707161
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