Effectiveness of Typhoid Vaccines
Direct Answer
Typhoid vaccines are highly effective in preventing typhoid fever, with the Vi polysaccharide vaccine preventing approximately 69% of cases in the first year and the oral Ty21a vaccine preventing about 50% of cases over three years, while the newer Vi-TT conjugate vaccine demonstrates 78% efficacy over four years and is now the WHO-preferred option for all ages. 1, 2, 3
Vaccine-Specific Efficacy Data
Vi Polysaccharide Vaccine (Injectable, Single Dose)
- First-year efficacy is 69% (95% CI: 63%-74%), representing the highest single-year protection among licensed vaccines 4
- Second-year efficacy drops to 59% (95% CI: 45%-69%), showing moderate waning of immunity 4
- Three-year cumulative efficacy is approximately 55% (95% CI: 30%-70%), though this is based on limited data from a single South African trial in the 1980s 1, 4
- Protects individuals aged 2 to 55 years based on trial data 4
Oral Ty21a Vaccine (Three-Dose Schedule)
- Three-year cumulative efficacy is 50% (95% CI: 35%-61%), providing moderate protection when all four doses are completed 4
- Protection remains consistent through five years of follow-up without significant waning 1
- Efficacy is lower with fewer doses: the four-dose regimen significantly outperforms three doses (95.8 vs 160.5 cases per 100,000 person-years, p<0.004) 5
- Effective in individuals aged 3 to 44 years 4
Vi-TT Conjugate Vaccine (Typbar-TCV, Single Dose)
- Four-year efficacy is 78.3% (95% CI: 66.3%-86.1%), demonstrating superior and durable protection compared to older vaccines 3
- Efficacy by age group: 70.6% in children 9 months to 2 years, 79.6% in children 2-4 years, and 79.3% in children 5-12 years 3
- Number needed to vaccinate is 163 to prevent one case of typhoid fever 3
- Approximately 96% of recipients seroconvert at 4-6 weeks, with 75.6% remaining seroconverted at 4 years 6
- Antibody decay is faster in children ≤2 years, though clinical protection remains substantial 6
Vi-rEPA Vaccine (Two-Dose Schedule)
- First-year efficacy is 94% (95% CI: 75%-99%) and second-year efficacy is 87% (95% CI: 56%-96%) 4
- Cumulative efficacy at 3.8 years is 89% (95% CI: 76%-97%), the highest among all typhoid vaccines 7
- Currently unlicensed and evaluated in only one trial in Vietnamese children aged 2-5 years 4, 7
Comparative Effectiveness
The Vi-TT conjugate vaccine (Typbar-TCV) is now the WHO-preferred vaccine due to its superior efficacy profile, single-dose convenience, and effectiveness in children as young as 6 months 2. The older Vi polysaccharide vaccine provides excellent first-year protection but wanes more rapidly 4. The oral Ty21a vaccine offers moderate, sustained protection but requires multiple doses and careful administration 4.
Important Clinical Considerations
- No typhoid vaccine provides 100% protection, and vaccination does not eliminate the need for careful food and water selection in endemic areas 2, 8
- Protection can be overwhelmed by large inocula of S. typhi 2
- Breakthrough infections with Vi-TT vaccine occur after a median of 3.4 years, suggesting potential need for booster doses 6
- Previous vaccination provides incomplete protection and does not protect against paratyphoid fever 8
Booster Requirements
- Vi polysaccharide vaccine requires boosters every 3 years for continued protection in persons with ongoing exposure 1, 9
- Oral Ty21a vaccine requires revaccination with the full four-dose series every 5 years 1, 9
- Vi-TT conjugate vaccine booster schedules are under investigation, with current data showing waning antibody levels but sustained clinical protection through 4 years 3, 6
Safety Profile
All typhoid vaccines demonstrate excellent safety profiles with mild, self-limited adverse events 1, 4. The oral Ty21a vaccine has adverse event rates comparable to placebo (<1/100,000 doses) 1. The Vi polysaccharide vaccine causes injection site reactions but rarely systemic effects 1. No serious adverse events have been reported in randomized controlled trials of any currently available typhoid vaccine 4.