Since its recognition in I969 as a novel, acute and often fatal haemorrhagic disease, it has become apparent that Lassa fever is both considerably more prevalent and rather less frequently fatal that was at first supposed. In the endemic areas of Sierra Leone where up to 40% of the population of some villages may show evidence of antibodies against Lassa virus, I0 to 15% of hospital medical admissions and 30% of hospital medical deaths have been found to be due to Lassa fcver (Mac Cormick et al., I98I). Crude estimates of the incidence and annual mortality in West and Central Africa as a whole (total population c. I48 million) suggest there may be some I00,000 cases of Lassa fever per year with around 5,000 fatalities (J . D. Frame, personal cummunicarion) .
The development of an effective vaccine would be clearly of benefit to the populations of the endemic regions of West-Africa, and also to the medical, nursing and other staff who may be called upon to care for patients from these areas, both in Africa and elsewhere in the world.
Here I wish to review the structure of the Lassa virus, and some aspects of its antigenic relationship with the non-pathogenic Mopeia virus, and briefly to discuss possible approaches to the development of a vaccine.