Protection against experimental cholera after immunization with a purified glutaraldehyde toxoid given subcutaneously (sc), orally, or by a combined sc-oral sequence was compared in dogs. The protection induced by toxoid appeared to be entirely due to antitoxin. Repeated sc immunization with precipitated toxoid resulted in 77% protection at two weeks but no protection at four months. Protection after sc immunization correlated with titers of antitoxin in serum and appeared to result entirely from serum-derived antibody. In contrast, immunization by the sc-oral sequence resulted in prolonged protection (74% at two weeks. P = 0.04; 100% after four months, P = 0.004; and 57% after eight months, P = 0.01) that was not related to the low serum antitoxin titers achieved. This sequence was effective only if the oral booster was given in divided doses. Immunization by oral priming and boosting was ineffective. These results suggest that the sc-oral sequence of immunization is an efficient means of stimulating the enteric immune response to nonreplicating protein antigens and may provide an improved approach to immunization against cholera.
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