Q. What is typhoid vaccine given for?
Current typhoid vaccines are monovalent vaccines that protect against typhoid fever or enteric fever. Some old types of multivalent vaccines were used to protect against typhoid fever as well as paratyphoid fever A, B & C. They are neither available anymore nor are desirable to be used.

Q. What are the types of typhoid vaccine?
There are two generations of typhoid vaccines, the old generation -whole cell killed vaccines & the newer generation vaccines including Vi antigen vaccine and oral ty21a vaccine.

Whole cell killed typhoid vaccines :- These first generation vaccines contain whole cell of killed typhoid bacilli. Initially polyvalent vaccines were available containing S. typhi & paratyphi A &B (TAB vaccine). As paratyphi A & B are not common & as the reactions were more common with paratyphi A & B components, they were dropped from the vaccine and the later vaccines were monovalent vaccines containing only S. typhi. It is either acetone or phenol inactivated vaccine. Accordingly you have acetone killed whole S. typhi vaccine or phenol killed whole cell S.typhi vaccine. Acetone killed S.typhi vaccine is better than phenol killed vaccine as it has more Vi antigen preserved during the process of inactivation. However, it is not available in the open market. Phenol killed vaccine was available in open market till 1-2 years back but currently even that is not available for use.

Vi antigen vaccine :- It contains the Vi polysaccharide antigen of S. typhi. As Vi polysaccharide is the most important virulent factor, the antibodies against Vi antigen are protective against typhoid fever. It is a subunit-inactivated vaccine. It is freely available for use.

Oral ty21a vaccine :- This is an oral live vaccine. It contains the ty21a strain of S. typhi, which is a mutant strain lacking UDP galactose epimerase enzyme. This enzyme is very important in completing the cell wall synthesis of bacilli. Once given orally the mutant bacilli will colonize the surface mucosal cells and start growing. However, as the cell wall synthesis is not completed, the bacilli will not be able to complete its formation and invade the mucosal cells. Instead they will break open and liberate the antigens to the gut associated lymphoid follicles. This will induce local gut immunity without any systemic immunity. This is enough to prevent enteric fever on further exposure to wild S.typhi bacilli.

Q. How are the typhoid vaccines available?
Whole cell killed typhoid vaccine is not available currently. It used to be available as multidose bulbs containing 5 ml i.e. 10 adult doses or 20 pediatric doses. The Vi antigen vaccine is available as single dose ready to use vials containing 0.5 ml of vaccine or as multidose bulbs containing 4-10 doses (2-5 ml ) depending on brand used. Some brands are available as 0.5 ml containing monodose prefilled syringes. The ty21a oral vaccine is available as a pack of 3 capsules.

Q. How are typhoid vaccines stored?
All typhoid vaccines are stored in refrigerator at 2-8⁰C. They should not be frozen. If frozen by mistake, they should be discarded as the efficacy goes down & the incidence of side effects goes up. It is important to stress to the patients that even the oral vaccine, though in capsule form should be kept in refrigerator (unlike many other medicinal capsules, which may be stored at room temperature)!