Two booster injections of vaccine of Sh. flexneri III 24 weeks after primary vaccination did not stimulate increases in protective power of sera from boys when a statistically significant level was still present. Individual and pooled sera did show slight increases in homologous agglutinative titers.

Three booster injections of vaccine of Sh. fiexneri VI 54 weeks after primary vaccination did stimulate statistically significant increases in protective power in the sera of boys when the protective power had decreased to zero. The increase persisted at least 6 weeks. Two of the three individual sera showed slight increases in homologous agglutinative titers within one week after the first booster injection.

Three booster injections of pentavalent Shigella vaccine 78 weeks after primary vaccination stimulated significant increases in protective power in the pooled serum of girls for the four component Shigellae against which the protective power had decreased to non-significant levels. These increases persisted at least 6 weeks. No significant increase developed in the protective power for the fifth homologous Shigella for which there was still a significant level at the time of the first booster injection. Increased agglutinative titers for each of the five component Shigellae were stimulated.

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This study was started under a contract with the Office of Scientific Research and Development and finished under a grant from the U. S. Public Health Service.

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