question and answer
February 1995
C. KENNETH DRESSER, MD, of Toronto, Ont., writes in to ask, "How effective is typhoid vaccine in preventing infection? If the injectable vaccine was given to someone 10 to 20 years ago or more, should the oral vaccine be given when available? Is it true that someone who has had a typhoid booster 20 years ago does not require any repeats?"
If it is appropriate for travel or other reasons, the patient inquestion should indeed be re-vaccinated. Until recently the only typhoid vaccines available were parenteral preparations of either heatphenol or acetone-inactivated Salmonella typhi. These vaccines gave modest protection but systemic and local reactions were quite common. Recently Ty21a, a strain of S. typhi attenuated by mutation, has been licensed for use as a live oral vaccine. Trials have been shown that it provides 65% protection for at least five years and is very safe. It's now the vaccine of choice for typhoid fever prevention, but it must be given as a series of three oral doses. Attempts to develop an effective single-dose oral vaccine and a safer parenteral preparation are under way. Administration of any typhoid vaccine should not supplant routine measures and common sense with respect to travellers' discretion about food and water sources. DM
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