1. the development, characterization, production, and certification of a primary seed virus for a cell culture 17D vaccine

2. research, preferably conducted in an existing YF vaccine production facility, leading to the development of production protocols for a cell culture YF vaccine

3. research to improve biological markers and reduce neurovirulence of the 17D vaccine and to define the dose response of the product on the basis of studies in man

4. formulation and evaluation of more satisfactory thermal stabilization agents for the currently available YF vaccine.

If chick embryo cell cultures are used as the substrate, they should be derived from embryonated eggs from a monitored, specific pathogen-free flock of chickens. Diploid cell culture products must meet WHO requirements for freedom from cellular DNA and should be used only if they have been used in the past for a live attenuated virus vaccine. The original seed virus should be a 17D derivative that is free from leucosis virus and that meets WHO requirements for a YF fever vaccine seed.

The thermal instability and relatively short shelf life of the live-attenuated YF vaccine probably could be corrected through inexpensive and straightforward investigations. Such investigations could provide a cheaper, longer lasting, and more abundant YF vaccine.

At present, basic information about the 17D virus strain is lacking. Basic research studies on the biological and biochemical properties of this virus strain should be conducted simultaneously with the investigations described above to provide a better understanding of genetic variation from passage in cell cultures. Particular emphasis should be given to defining genetic markers useful for the characterization of cell culture derived vaccines. The availability of in vitro and in vivo markers for the characterization of the cell-culture adapted YF seed to be used for vaccine production is vitally important, and several marker systems should be developed for this purpose. The markers should be shown to be reliable and reproducible, and each passage of virus should be monitored for changes.

Currently, YF 17D vaccines are not recommended for use in infants under 6 months of age, and some countries do not require the International Certificate of Vaccination under 1 year of age. The incidence of encephalitis following YF 17D vaccination of infants is not known with any certainty and should be studied further. In one analysis, the incidence was estimated to be at least 0.3 percent in infants under 6 months of age (Louis et al., 1981). Because immunization of infants younger than 6 months of age is desirable in many circumstances, reduction of the existing level of neurovirulence of 17D vaccine should be a goal of YF research.

Such a research project could produce a vaccine ready for initial trials in humans in about 2 years, depending to some extent on the amount of vaccine and seed virus testing in monkeys. In any event, vaccine testing in humans is not expected to be a special problem.