An Inactivated Cell-Culture Vaccine against Yellow Fever

BACKGROUND Yellow fever is a lethal viral hemorrhagic fever occurring in Africa and South America. A highly effective live vaccine (17D) is widely used for travelers to and residents of areas in which yellow fever is endemic, but the vaccine can cause serious adverse events, including viscerotropic disease, which is associated with a high rate of death. A safer, nonreplicating vaccine is needed. METHODS In a double-blind, placebo-controlled, dose-escalation, phase 1 study of 60 healthy subjects between 18 and 49 years of age, we investigated the safety and immunogenicity of XRX-001 purified whole-virus, beta-propiolactone-inactivated yellow fever vaccine produced in Vero cell cultures and adsorbed to aluminum hydroxide (alum) adjuvant. On two visits 21 days apart, subjects received intramuscular injections of vaccine that contained 0.48 mu g or 4.8 mu g of antigen. Levels of neutralizing antibodies were measured at baseline and on days 21, 31, and 42. RESULTS The vaccine induced the development of neutralizing antibodies in 100% of subjects receiving 4.8 mu g of antigen in each injection and in 88% of subjects receiving 0.48 mu g of antigen in each injection. Antibody levels increased by day 10 after the second injection, at which time levels were significantly higher with the 4.8-mu g formulation than with the 0.48-mu g formulation (geometric mean titer, 146 vs. 39; P<0.001). Three adverse events occurred at a higher incidence in the two vaccine groups than in the placebo group: mild pain, tenderness, and (much less frequently) itching at the injection site. One case of urticaria was observed on day 3 after the second dose of 4.8 mu g of vaccine. CONCLUSIONS A two-dose regimen of the XRX-001 vaccine, containing inactivated yellow fever antigen with an alum adjuvant, induced neutralizing antibodies in a high percentage of subjects. XRX-001 has the potential to be a safer alternative to live attenuated 17D vaccine.