Typhoid Conjugate Vaccine
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Direct answer
Typhoid conjugate vaccine (TCV) links the Vi polysaccharide of Salmonella Typhi to a protein carrier, which allows protection from about six months of age. The older Vi polysaccharide vaccine does not give a reliable response in children under two years, and that age difference is the key comparison in exams.
What you must remember
- Vi polysaccharide vaccine: plain Vi antigen, usable from two years of age, with protection that wanes and needs periodic boosters.
- TCV: Vi antigen conjugated to a protein carrier, such as tetanus toxoid in Indian products, usable from six months.
- Memory response: conjugation converts a weak polysaccharide response into one that builds immunological memory in infants.
- Dose: a single intramuscular dose; the IAP does not currently recommend a routine booster, so follow the latest national guidance.
- IAP advice: the Indian Academy of Pediatrics recommends TCV from six months, within a routine schedule.
- Other typhoid vaccine: Ty21a is an oral live attenuated vaccine, which is different from the injectable TCV.
- Contraindication: a severe allergic reaction to a previous dose or a vaccine component.
- Deferral: moderate or severe acute illness is a reason to postpone vaccination.
Choosing the typhoid vaccine by age
A 9-month-old is due for typhoid protection in an area where enteric fever is common, and the clinic stocks both products. Age decides the choice, because the polysaccharide vaccine is not suitable for an infant.
| Feature | Vi polysaccharide | TCV |
|---|---|---|
| Antigen | Vi polysaccharide alone | Vi conjugated to protein |
| Earliest age | Two years | Six months |
| Memory response | No | Yes |
| Booster approach | Periodic | None routinely advised by IAP |
Conjugation is the reason an infant can respond to the Vi antigen at all. Once a child is two years or older, either product may be used, and the programme's supply usually decides which.
Enteric fever causes prolonged fever, abdominal symptoms and headache, and it spreads through contaminated food and water. Vaccination adds protection, but it does not replace safe water, sanitation and food hygiene. Both interventions are needed for a durable reduction in disease.
A vaccinated child can still develop enteric fever, and its features are often nonspecific. A febrile child is therefore investigated as usual, and vaccination status does not exclude typhoid.
Where the exam tries to catch you
Typhoid vaccine questions usually turn on age, product type and the difference from the oral vaccine.
- Trap: giving Vi polysaccharide to a 10-month-old because it is the familiar product Correct: it does not give a reliable response under two years, so TCV is the choice at that age.
- Trap: confusing TCV with the oral live vaccine Correct: TCV is an injectable conjugate, while Ty21a is an oral live attenuated vaccine.
- Trap: assuming one dose gives lifelong typhoid protection Correct: protection may wane over years, and the IAP currently advises no routine booster, so follow the latest national guidance.
Frequently asked questions
From what age can typhoid conjugate vaccine be given?
Typhoid conjugate vaccine can be given from six months of age, which is earlier than the Vi polysaccharide vaccine allows. This matters for infants in areas where enteric fever is common.
Why does the Vi polysaccharide vaccine not suit infants?
The Vi polysaccharide vaccine does not give a reliable response in children under two years. It lacks a protein carrier, so it does not stimulate the memory response that typhoid conjugate vaccine provides.
How does typhoid conjugate vaccine differ from Ty21a?
Typhoid conjugate vaccine is an injectable Vi antigen conjugate, while Ty21a is an oral live attenuated vaccine. Both protect against enteric fever, but they differ in route, live status and the age at which each is licensed.
Does typhoid vaccination replace hygiene measures?
Typhoid vaccination does not replace safe water, sanitation and food hygiene. Vaccines reduce the disease burden, but enteric fever continues to spread through contaminated food and water.
What contraindication applies to typhoid conjugate vaccine?
A severe allergic reaction after a previous dose of typhoid conjugate vaccine, or to one of its components, contraindicates repeating it. Moderate or severe acute illness is a reason to defer the vaccine, as with other childhood vaccines.