A new conjugate typhoid vaccine suitable for administration to infants and young children was efficacious, highly immunogenic, and well tolerated, compared with placebo, in a phase 2 study that tested the vaccine using a human typhoid infection model.
In a study that compared two formulations of typhoid vaccine to a control meningococcal vaccine, the new Vi-conjugate (Vi-TT) vaccine had an efficacy of 54.6% (95% confidence interval, 26.8-71.8) and a 100% seroconversion rate.
The study was not powered for a direct comparison of the efficacy of the Vi-TT with the efficacy of the Vi-polysaccharide (Vi-PS), the other vaccine used in the study. The Vi-PS vaccine had an efficacy of 52.0% (95% CI, 23.2-70.0), and 88.6% of the Vi-PS recipients had seroconversion.
However, “clinical manifestations of typhoid fever seemed less severe among diagnosed participants following Vi-TT vaccination,” Celina Jin, MD, and her colleagues wrote (Lancet. 2017 Sep 28: doi: 10.1016/S0140-6736[17]32149-9). Fever, defined as an oral temperature of 38° C or higher, was seen in 6 of 37 (16%) Vi-TT recipients, 17 of 31 (55%) receiving control, and 11 of 35 (31%) receiving Vi-PS.
Geometric mean titers also were significantly higher in the Vi-TT group than in the Vi-PS group, with an adjusted geometric mean titer of 562.9 EU/mL for Vi-TT and 140.5 EU/mL for Vi-PS (P less than .0001).
The study enrolled 112 healthy adult volunteers who were randomized 1:1:1 to receive Vi-PS, Vi-TT, or control meningococcal vaccine. A total of 103 of the participants eventually received one of the two study vaccines or the control vaccines, and that group was included in the per-protocol analysis.
After vaccination (recipients and investigators were masked as to which formulation participants received), study participants kept an online diary to report any vaccination-related symptoms for 7 days, and also had clinic visits scheduled at days 1, 3, 7, and 10.
Participants received one oral dose of wild-type Salmonella enterica serovar Typhi Quailes strain bacteria about 1 month after vaccination. The dose was 1-5x104 colony forming units, and was administered immediately following a 120-mL oral bolus of sodium bicarbonate (to neutralize stomach acid).
Participants then were seen daily in an outpatient clinic for 2 weeks. At each visit, investigators monitored vital signs, performed a general assessment, and drew blood to assess for typhoid bacteremia. Participants also kept an online diary for 21 days, reporting twice-daily self-measured temperatures as well. No antipyretics were allowed before typhoid diagnosis.
Participants who met the study’s criteria for typhoid diagnosis were treated with a 2-week course of ciprofloxacin or azithromycin; patients who did not become ill were treated 14 days after the oral typhoid challenge. None of the four serious adverse events reported during the study was deemed to be related to vaccination.
CDC/Armed Forces Institute of Pathology, Charles N. Farmer
Histopathology of a lymph node in a case of typhoid fever.
That broad definition of typhoid infection was used to determine attack rates for the study’s primary outcome measure. However, Dr. Jin and her colleagues also looked at a less stringent – and perhaps more clinically pertinent – definition of 12 hours of fever of 38° C or higher followed by S. Typhi bacteremia. Using those criteria, the Vi-TT vaccine prevented up to 87% of infections.
Salmonella Typhi is the world’s leading cause of enteric fever, said Dr. Jin, of the Oxford Vaccine Group at the University of Oxford (England). Up to 20.6 million people per year are affected, with children most commonly infected and low-resource populations in Asia and Africa hardest hit.
Both prescription and over-the-counter antibiotics are used worldwide to combat typhoid fever, and S. Typhi strains are becoming increasingly antibiotic resistant in South Asia and Africa, Dr. Jin and her coauthors said.
The typhoid vaccines that are currently licensed are either not suitable for administration to infants and young children, or are insufficiently immunogenic in younger populations.
The typhoid conjugate vaccine used in the study combines the Vi-polysaccharide capsule with a protein carrier, increasing host immunologic response and making the vaccine effective in infancy.
“This human challenge study provides further evidence to support the deployment of Vi-conjugate vaccines as a control measure to reduce the burden of typhoid fever, because those individuals living in endemic regions should not be made to wait another 60 years,” wrote Dr. Jin and her coauthors.
The study was funded by the Bill & Melinda Gates Foundation and the European commission FP7 grant, Advanced Immunization Technologies.