Typhoid fever has been reported from all parts of the world. With the improvement in sanitation and standard of living, the disease is now uncommon in developed countries. However, in developing and under-developed countries, it is still one of the major infectious disease having significant mortality and morbidity. It has been estimated that globally about 16 million cases of typhoid fever are reported annually causing about 6,00,000 deaths Crump J.A. et al.(2004).In addition to this, the socio-economic implications of the disease are huge because of following reasons :-
1. Complete recovery takes several weeks.
2. Emergence of multi-drug resistant strains contributing to escalating cost of treatment. It may also contribute to higher morbidity and increased mortality.
3. Recent research reveal that the INFANTS and children up to 5 years of age suffer most from typhoid fever than in any other age group.
A large population particularly infants remained unprotected against typhoid fever Since there was no vaccine available for this age group and children less than 2 years of age.
Spread of the disease can be controlled by improving sanitation, water supply and standard of living, however, this is almost impossible to implement in many of the countries including India because of financial , social and technical reasons. Under these circumstances, the ideal way is to provide through active immunization.
Introduction of Vi polysaccharide typhoid vaccine with much lower reactogenicity. has led to marked increase in acceptance of vaccination as a preferred method to prevent typhoid fever. Still a large population particularly infants remained uncovered since there was no vaccine available for children and infants less than 2 years of age.
Further the Vi antigen includes immunity for about 2-3 years only, thus revaccination is required.
Conjugated Vi polysaccharide typhoid vaccine can be used to vaccinate 3 month or older infants with very high levels of protection. With Vi conjugated vaccine, it is expected that immunity will last for much longer period due to establishment of memory, and immunity may be boosted by natural exposure to S.typhi.