742 family contacts of 149 cholera patients with bacteriologically confirmed Vibrio cholerœ Inaba infection were given either placebo (vaccine diluent) or monovalent Inaba vaccine of proven efficacy within 12 hours of hospital admission of the index case. Rectal swabs and histories of diarrhœa obtained daily for 10 consecutive days revealed no appreciable difference in subsequent rates of infection, date of onset of positive stools, duration of vibrio shedding, or clinical expression of the disease. Although protection did seem to begin by the 9th day of observation, the numbers were too small to be statistically significant. Given the close temporal clustering of secondary infections among family contacts near the onset of the index case (well over 95% had occurred by day 8), tetracycline remains the prophylactic agent of choice.
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