Conjugate typhoid vaccine safe and effective in phase 2 trials

The Oxford Vaccine Group has developed a typhoid challenge model that provides an important bridge in clinical testing and affords the possibility of significant acceleration of the vaccine development process. Despite the controversy human challenge models sometimes engender, previous human typhoid challenge studies contributed to the development of the live attenuated typhoid vaccine Ty21a.

The conjugate vaccine tested by Dr. Jin and her colleagues is a much-needed weapon in the public health armamentarium of typhoid control. Treatment options are limited in regions of South Asia and Africa where endemic typhoid shows increasing antibiotic resistance.

This human challenge study provides the first evidence that the conjugate vaccine reduces the attack rate of typhoid fever, though its use in India has shown it to be safe and immunogenic, even in children as young as 6 months of age.

The stringent definition of typhoid fever attack used in this study may result in a finding of lower efficacy than would be seen in a field trial, and a National Institutes of Health–sponsored study of another conjugate vaccine found efficacy rates of 89% among Vietnamese preschoolers followed for nearly 4 years after vaccination. When the present study’s data were reanalyzed with use of the less stringent case definition of fever followed by typhoid bacteremia, a similar efficacy of 87.1% was seen for the conjugate vaccine. A larger sample size would be needed in a challenge study that included the less stringent definition as a coprimary endpoint, but results might better correlate with real-world field trials.

Phase 3 and 4 trials for the typhoid conjugate vaccine are forthcoming, but final results will not be tallied for many years. The typhoid challenge study reported by Dr. Jin and her colleagues bolsters hopes that the candidate vaccine will help with typhoid control where it’s most needed.

Nicholas A. Feasey, MD , is at the Liverpool (England) School of Tropical Medicine. Myron M. Levine, MD , is at the University of Maryland, Baltimore. Their comments were drawn from an editorial accompanying the study (Lancet. 2017 Sep 28. doi: 10.1016/S0140-6736[17]32407-8 ).