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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 growth characteristics have been attenuated for humans, two recipients of the 45AZ5 virus developed unmodified acute dengue fever. Viremia was observed within 24 hr of inoculation and lasted 12 to 19 days. Virus isolates from the blood produced large plaques in cell culture and showed diminished temperature sensitivity. The 45AZ5 virus is unacceptable as a vaccine candidate. This experience points out the uncertain relationship between in vitro viral growth characteristics and virulence factors for humans.