A single dose vaccine formulation for use in the prevention of typhoid fever caused by Salmonella typhi, comprising a capsular Vi-polysaccharide (ViPs) of Salmonella typhi conjugated to a carrier protein of tetanus toxoid as vaccine antigen, wherein a single dose of the vaccine formulation is sufficient to elicit the required T-dependent immune response against S. typhi including in children below 2 years of age, wherein the vaccine comprises 5μg to 25μg of said Salmonella typhi Vi-polysaccharide-tetanus toxoid conjugate vaccine antigen obtained by a method comprising: a. Fed-batch mode of cultivation to obtain purified ViPs with a feed medium, the said feed medium comprising feeding with a solution containing glucose at a range of 1 to 2 mg/mL concentration at a pH maintained in the range of 6.90 to 7.20 and dissolved oxygen level maintained between 40%-60%, wherein ammonia solution (50%) was supplied as a nitrogen source along with the feed medium; b. treating the purified ViPs in the presence of sodium bicarbonate to obtain partially de-O-acetylated ViPs; c. treating the purified ViPs of step (b) with a cross linking agent EDAC; d. activating the ViPs of step (c) with a linker molecule ADH in presence of EDAC; e. treating the activated ViPs linked to a linker molecule ADH of step (d) at a concentration of 1 mg/ml to 5 mg/ml of purified ViPs of ∼900 kDa or at a concentration of 5mg/ml to 7.5 mg/ml of purified ViPs of ∼250 kDa with a tetanus toxoid as carrier protein in the presence of EDAC, enabling the -NH 2 present at the other end of the ADH molecule to bind with the -COOH group of the carrier protein molecule, thereby forming an effective bridge between the ViPs and the carrier, to form the Vi-polysaccharide-tetanus toxoid conjugate; f. diafiltering through continuous buffer exchange with phosphate buffered saline of the Vi-polysaccharide-carrier protein conjugate of step (e) with a 1000 kDa membrane to obtain the purified ViPs-tetanus toxoid vaccine antigen. a. Fed-batch mode of cultivation to obtain purified ViPs with a feed medium, the said feed medium comprising feeding with a solution containing glucose at a range of 1 to 2 mg/mL concentration at a pH maintained in the range of 6.90 to 7.20 and dissolved oxygen level maintained between 40%-60%, wherein ammonia solution (50%) was supplied as a nitrogen source along with the feed medium; b. treating the purified ViPs in the presence of sodium bicarbonate to obtain partially de-O-acetylated ViPs; c. treating the purified ViPs of step (b) with a cross linking agent EDAC; d. activating the ViPs of step (c) with a linker molecule ADH in presence of EDAC; e. treating the activated ViPs linked to a linker molecule ADH of step (d) at a concentration of 1 mg/ml to 5 mg/ml of purified ViPs of ∼900 kDa or at a concentration of 5mg/ml to 7.5 mg/ml of purified ViPs of ∼250 kDa with a tetanus toxoid as carrier protein in the presence of EDAC, enabling the -NH 2 present at the other end of the ADH molecule to bind with the -COOH group of the carrier protein molecule, thereby forming an effective bridge between the ViPs and the carrier, to form the Vi-polysaccharide-tetanus toxoid conjugate; f. diafiltering through continuous buffer exchange with phosphate buffered saline of the Vi-polysaccharide-carrier protein conjugate of step (e) with a 1000 kDa membrane to obtain the purified ViPs-tetanus toxoid vaccine antigen.