Prospective study of the association between serum antibodies to lipopolysaccharide O antigen and the attack rate of shigellosis

A means for determining immune status against shigellosis would significantly improve the design and evaluation of interventional and other epidemiologic studies. Previous case-control studies have indicated the potential role of humoral antilipopolysaccharide antibodies. To test this proposition, 190 soldiers serving in a field unit were monitored prospectively for 2.5 months for shigellosis. Blood samples were taken at the beginning of the follow-up period and tested for serological evidence of prior exposure to Shigella sonnei and Shigella flexneri. The risk for acquiring S. sonnei shigellosis was 3.7 times higher for individuals lacking homologous antibodies (P < 0.02). The risk for acquiring S. flexneri shigellosis was 2.4 times higher for individuals lacking antibodies, although a low attack rate for S. flexneri resulted in numbers too small to achieve statistical significance. While the importance of the serum antilipopolysaccharide antibodies in protection against the disease remains unclear, these findings demonstrate that they are strong markers of acquired immunity. Serological markers should be incorporated in epidemiologic studies of shigellosis and in the design and evaluation of future trials of potential anti-Shigella vaccines.