Typhoid fever is a life-threatening bacterial infection caused by Salmonella Typhi. This illness is typically contracted by consuming food or water contaminated with the feces of an infected person, a transmission route known as the fecal-oral pathway. Symptoms include a prolonged high fever, fatigue, stomach pain, and headache, which can lead to severe complications like intestinal perforation if left untreated. Vaccination is a proven preventative measure, especially for people traveling to or living in areas where sanitation is limited and the disease is common.
The Two Main Types of Typhoid Vaccines
Protection against typhoid fever is delivered through two widely available vaccine types, each administered differently to stimulate an immune response. The first is an injectable vaccine known as the Vi capsular polysaccharide vaccine (ViPS). This vaccine uses a purified component from the outer coat of the Salmonella Typhi bacterium, called the Vi antigen, to prepare the immune system for a future infection.
The ViPS vaccine is administered as a single intramuscular shot and is approved for use in individuals two years of age and older. Since it contains only a subunit of the bacteria, it is often referred to as an inactivated or subunit vaccine.
The second type is an oral formulation called the live-attenuated Ty21a vaccine. The Ty21a vaccine consists of four separate capsules taken on alternating days over a one-week period. This oral vaccine uses a weakened, or attenuated, strain of the live Salmonella Typhi bacteria that can provoke an immune response without causing the disease. Because it is a live vaccine, it is generally approved for use in people six years of age and older.
Duration of Protection and Effectiveness Rates
The duration of protection varies significantly between the two types, making the choice dependent on an individual’s exposure risk and travel plans. The injectable Vi capsular polysaccharide vaccine typically offers a shorter duration of protection. For most travelers, a single dose is considered effective for approximately two years.
Some data suggests that protective immunity from the ViPS vaccine may last closer to three years for people with continued exposure, but the two-year interval is the standard recommendation. The effectiveness of the ViPS vaccine is generally reported to be in the range of 50% to 80%. Studies have shown that its efficacy is highest immediately after vaccination, preventing about 69% of cases in the first year, which may decline slightly to around 59% in the second year.
In contrast, the oral live-attenuated Ty21a vaccine provides a significantly longer period of protection after the primary four-dose series is completed. The efficacy of this vaccine is shown to persist for at least five years in most recipients.
The overall effectiveness of the oral Ty21a vaccine is comparable to the injectable version, often cited in the 50% to 80% range. One enteric-coated capsule formulation was found to confer 62% protection over a period of seven years in long-term surveillance studies. Therefore, the choice between the two often balances the convenience of a single shot against the longer duration of protection offered by the four-dose capsule series.
Guidance on Revaccination and Boosters
Maintaining immunity requires adherence to specific revaccination schedules once the initial protective period for each vaccine type has passed. The injectable ViPS vaccine necessitates a booster dose every two years if an individual remains at risk of exposure to Salmonella Typhi. This schedule applies to travelers who frequently visit endemic areas or laboratory workers who handle the bacteria.
The booster for the ViPS vaccine is a single intramuscular injection, which refreshes the immune system’s memory of the Vi antigen. For the oral Ty21a vaccine, the process for revaccination is different because the entire series must be repeated.
Individuals who have completed the initial four-dose regimen and continue to face exposure should repeat the full four-capsule series every five years. The recommendation to repeat the full series is based on efficacy studies showing that protection lasts at least five years.
Neither vaccine offers 100% protection against typhoid fever. Therefore, individuals, even if vaccinated, should continue to take necessary precautions, such as ensuring safe food and water consumption, especially when traveling. Consulting a healthcare provider before travel is advisable to determine the most appropriate vaccine and schedule based on the destination and anticipated duration of exposure.