Q. What is the dose of the typhoid vaccine?
Whole cell killed typhoid vaccine is given in the dose of 0.25 ml per dose for< 10 year old children & 0.5 ml for> 10 year old patients. It is given subcutaneously over thigh or arm region. Two doses are given at the interval of 4 weeks. In case of emergency, like people travelling to endemic areas in short time or during epidemics of typhoid fever, the interval between 2 doses can be reduced to 15 days. This vaccine can also be given intradermally in the dose of 0.1 ml.

Vi antigen vaccine is given in the dose of 0.5 ml intramuscularly either on anterolateral aspect of thigh or in deltoid muscle. It is given as a single dose.

Oral ty21a vaccine is a course of 3 capsules given orally on alternate days. The capsules should be swallowed intact & not opened or chewed. In most, the pack contains 4 capsules and such 4 capsules are given on alternate days. Liquid form of oral ty21a vaccine in sachet form is more effective than the capsule form. But sachet form is not available in India.

When is the typhoid vaccine given?
Typhoid fever can occur at any age. 10% of cases occur in infants < 1 year of age. The most common group to be affected is school going children and adolescents. Hence earlier the vaccine is given better it is.

Q. What is the immune response to typhoid vaccine?
The protective antibodies are formed by 15-30 days following vaccination. Anti Vi antibodies are one of the important protective antibodies. Anti H and anti O antibodies are not protective. Oral ty21a vaccine induces local gut immunity, which prevents colonization and invasion when exposed to S.typhi subsequently.

The protective efficacy lasts for 2-3 years in most of the vaccinees following 2 doses of whole cell killed vaccine, 1 dose of Vi vaccine or 3 doses of ty21a vaccine. If one gives 4 capsules of oral ty21a vaccine, it will lead to protection for 3-5 years. After 2nd or 3rd year of vaccination, the protective efficacy falls to less than 50%.

As all the typhoid vaccines contain one polysaccharide antigen, they do not induce T cell dependent immune response. Hence, the immunity lasts for a short period of 3-5 years and there is no long lasting effect either with subclinical infection or by repeated vaccination. Hence one has to revaccinate every 3 years.

Q. On what does the typhoid efficacy depend?
Vaccination induces protective antibodies. But occurrence of typhoid fever also depends on endemicity in the area, host factors and the quantum of infecting dose. Hence, the clinical efficacy of vaccines does not depend only on the presence of antibodies or its titres. Unlike other diseases like Hib or Hepatitis B, the titres of antibodies after typhoid vaccine do not correlate well with its clinical efficacy. That is the reason why studies on typhoid vaccine talk about its clinical efficacy in field trials and not on seroconversion or antibody titres. One should only rely on data on clinical efficacy in field trials and not on antibody titres while conducting a comparative study on typhoid vaccines.

Q. When is a booster dose of typhoid vaccine given?
As discussed before, typhoid vaccines do not induce T cell memory. The protection following vaccines lasts for 3-5 years. Hence the vaccine needs to be repeated every 3-5 years. As there is no T cell memory the repeat course dose not "boost" immunity. Therefore one needs to repeat full course of vaccine every 3-5 years. In true sense, it is revaccination & not booster every 3-5 years.

Q. What is the efficacy of typhoid vaccines?
The clinical efficacy of all the typhoid vaccines is similar. It is 70 - 90 % with most of the vaccines. The phenol killed whole cell typhoid vaccine though is not as good as other 3 vaccines i.e. acetone killed vaccine, Vi antigen vaccine or the ty21a oral vaccine. The immunity lasts for 3-5 years as discussed before. The liquid form of ty21a vaccine in sachet form has efficacy as high as 90% in some studies. But it is not available at present. 4 capsule schedule of ty21a vaccine leads to 4-5 years protection as compared to 3-4 years with 3 capsule schedule.