The levels of protection (60% to 90% reduction in egg output and/or worm burdens) are higher than those required to provide full disease control, as predicted by epidemiological analyses and mathematical modelling (2, 62). Developing equally effective recombinant versions of these vaccines, however, is proving elusive. It is suggested that post-translational processing and, in particular, glycosylation, is crucial (29). Further research is being devoted to these issues and it is expected that improved expression systems will become available (54).