The yellow fever vaccine contains attenuated live viruses. Fifty-eight pregnancies followed by the European Network of Teratology Information Services (ENTIS) gave no substantial indication of embryo-or fetotoxic effect after first-trimester vaccination (Robert 1999). In a small number of women who received this vaccine during various stages of pregnancy there was no evidence of transplacental passage of the attenuated virus, although in most mothers who produced neutralizing antibodies these antibodies either crossed the placenta or were transferred to neonates through the colostrum. No adverse effects associated with prenatal exposure to this vaccine were observed at birth or during a 3- to 4-year follow-up period (Nasidi 1993). There is one report on congenital infection after first-trimester vaccination (Tsai 1993), but no other report has confirmed this potential risk. Finally, a small casc-control study conducted in Brazil (Nishioka 1998) compared 39 women who attended a hospital after spontaneous abortion with 74 women attending the antenatal clinic of the same hospital. A non significant risk for spontaneous abortion was found.
Recommendation. Yellow fever vaccine should be avoided during pregnancy. However, since yellow fever can be life-threatening, a pregnant woman should be immunized - even in the first trimester - if a trip to a region where the disease is endemic cannot be postponed.
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