Two different human vaccine trials examined interference arising from sequential administration of vaccines against heterologous alphaviruses. The first trial indicated that persons previously vaccinated against Venezuelan equine encephalitis virus (VEEV) exhibited poor neutralizing antibody responses to a live attenuated chikungunya virus (CHIKV) vaccine (46% response rate). The second trial prospectively examined neutralizing antibody responses to live attenuated VEEV vaccine in persons previously inoculated with either CHIKV vaccine or placebo. Following seroconversion to CHIKV, CHIKV vaccine recipients' geometric mean titers (GMTs) to VEEV by 80% plaquereduction neutralization titration never exceeded 10, compared with a peak GMT of 95 after VEEV vaccination for alphavirus-naive volunteers who initially received placebo (P < .003). ELISA antibody responses demonstrated cross-reactive IgG to VEEV after primary CHIKV immunization and then an anamnestic response upon subsequent VEEV vaccination. These data indicate that preexisting alphavirus immunity in humans interferes with subsequent neutralizing antibody response to a live attenuated, heterologous vaccine.