A dengue type 1, candidate live virus vaccine (45AZ5) was prepared by serial virus passage in fetal rhesus lung cells. Infected cells were treated with a mutagen, 5-azacytidine, to increase the likelihood of producing attenuated variants. The vaccine strain was selected by cloning virus that produced only small plaques in vitro and showed reduced replication at high temperatures (temperature sensitivity). Although other candidate live dengue virus vaccines selected for similar ...