Immune responses in Vietnamese children after a single dose of the auxotrophic, live Shigella flexneri Y vaccine strain SFL124.

ABSTRACT

The live, auxotrophic Shigella flexneri vaccine strain SFL124 was given in a single dose of 10(7), 10(8) or 10(9) colony forming units (cfu), respectively, to each of three groups of 10 Vietnamese children aged 9-14 years. The vaccine was well tolerated by all the children without any severe side effects such as diarrhoea or fever being observed. Mild symptoms were reported by five children. Only five children were found by culture to excrete SFL124 but, by PCR, 28 of 30 children were found to excrete the vaccine strain for up to 5 days (mean 2.8 days) with insignificant differences among the groups. Local mucosal immune responses and antibody secreting cell (ASC) responses to S. flexneri lipopolysaccharide (LPS) and invasion plasmid-coded antigens (Ipa) were elicited in the children in a dose-dependent manner. Doses of 10(9) cfu induced most prominent responses, followed by those of 10(8) and 10(7) cfu. The sIgA responses were the highest whereas the ASC were modest. High titres of serum antibodies to Shigella LPS and Ipa were found in all the children before ingestion of the vaccine which elicited increases in serum antibody titres in only a few of them. The immune response patterns seen indicate a booster rather than a primary response and may be a consequence of the endemic nature of shigellosis in Vietnam.