The present status of rotavirus vaccine development.

Rotavirus infection is one of the most important causes of morbidity in young children throughout the world. The high associated mortality in Southeast Asia (and elsewhere) warrants the development of a vaccine. It is probable that most of the life-threatening watery diarrhoea due to rotavirus infection occurs as a result of primary infection in children aged 6-18 months after protection due to maternal antibody has diminished. Thus rotavirus vaccines are targeted at young infants from birth to 3 months of age. At present three candidate rotavirus vaccines (RIT-4237, MMU-18007, WC3) have undergone trials in young children. A bovine rotavirus strain (RIT-4237), was shown to be safe, immunogenic and efficacious in prevention of severe rotavirus diarrhoea in young children in Finland. However it was found to be weakly immunogenic in infants in developing countries, and to have only low efficacy in prevention of disease. A simian rotavirus strain (RRV, MMU-18006) has proved to be highly immunogenic and its reactinogenicity to be diminished by pre-existing maternal antibody (in infants aged 1-4 months). It has high efficacy against clinically severe rotavirus infection. However protection is homotypic against human serotype 3 only so that eventually a multivalent vaccine incorporating reassortant rotavirus strains that protect against human serotypes 1, 2, 4 (and other newer serotypes) may be required. It is hoped that, once safe immunogenic and protective candidate rotavirus vaccines are identified, they can be administered in an acceptable form with no alteration to existing immunization schedules.

The current status of cholera vaccine development and experience with cholera vaccine trials in volunteers.

In recent years notable advances have been made in the development of improved vaccines to prevent cholera. These new vaccines are administered orally to maximally stimulate intestinal secretory immunity. Killed vibrios, given in conjunction with purified B subunit or administered alone, in three spaced doses, caused no adverse reactions and have conferred significant protection in volunteer challenge studies and in field trials. Two attenuated mutants of V. cholerae, prepared by recombinant DNA techniques, CVD 103 and CVD 103-HgR are well-tolerated and elicit prominent immune responses and protective immunity after ingestion of a single oral dose. Other modern approaches being pursued include the development of auxotrophic strains and of modifying attenuated S. typhi strain Ty21a to express V. cholerae Inaba and Ogawa LPS antigens.

The current status of typhoid vaccine development and clinical trials with typhoid vaccines.

The killed whole cell typhoid vaccines, although protective, have limited usefulness because of the adverse reactions they evoke. In contrast, new typhoid vaccines protect without reactogenicity. Attenuated oral vaccine Ty2la has been evaluated in field trials of efficacy in Santiago, Chile. Three doses of Ty2la in an enteric-coated formulation given within one week provided 69% efficacy for at least four years. Ty2la has reached the stage of being a practical public health tool. Two double auxotrophic (Aro-, Pur-) mutant strains of S. typhi (541Ty and 543Ty) were well-tolerated and stimulated cell-mediated immune responses but evoked little serological response. Parenteral purified Vi polysaccharide of S. typhi (single 25 mcg dose) was safe and immunogenic and provided 64-72% protection (for at least 17-21 months) in controlled field trials in Nepal and South Africa.