Progress in Obtaining a Better Yellow Fever Vaccine

Yellow fever is an affliction caused by a potentially lethal viral (flavivirus) hemorrhagic (causes bleeding) virus common in parts of Africa and South America. A highly effective vaccine made from live virus (known as the “17D vaccine”) is currently used to inject persons to prevent yellow fever; this vaccine is known to rarely cause serious adverse effects, namely, onset of allergic reactions, or a life-threatening or fatal infection that resembles yellow fever.

So, there is need for a safer (“nonreplicating”—in other words, not based on live virus) vaccine. In a recent article, “An Inactivated Cell-Culture Vaccine against Yellow Fever,” Thomas Monath, MD and his coauthors described their experience with a potentially safer vaccine (NEJM, 2011;364:1326-33). In their study, they evaluated the safety and effectiveness (in raising protective antibodies against yellow fever virus in recipients of the new vaccine) compared against placebo (i.e., no effective antigen present). The antigen (substance that elicits the immune response, which is the creation of protective antibodies) in the new vaccine was yellow fever vaccine produced in cell cultures utilizing inactivated (not live) and purified virus components.

The new vaccine created an acceptable immune response (protection) against the yellow fever virus, and side effects (occasional pain and tenderness at the site of injection) were not serious. This makes good sense, as the cell culture vaccine does not contain egg proteins and gelatin that might cause allergic reactions, and also does not allow the virus to replicate in the recipient. Two injections were necessary to create the protective immune response, which is similar to immunization regimens used for other diseases, such as Japanese encephalitis. It is not yet known if immunization with this new vaccine provides long-term protection, as does the current live vaccine. We will all eagerly await more information on this new vaccine.