Typhoid vaccine proves highly immunogenic, and could halve infection rate
A new typhoid vaccine has proven safe, highly immunogenic and could prevent more than half of typhoid infections according to a new study published in The Lancet. The study is a phase 2b trial of 112 adults and provides the first efficacy data for the leading candidate vaccine being considered for widespread use in children under 2 years, who are disproportionately affected by typhoid.
The trial uses a controlled human infection model, in which healthy volunteers are vaccinated and then deliberately exposed to the pathogen. These types of studies have been used to support the development of various vaccines (including the licenced cholera vaccine) as they can be rapidly deployed to assess vaccine efficacy.
The Vi-conjugate vaccine studied in this trial is only licensed for use in children under 2 years in India, and there are no typhoid vaccines licensed worldwide for use in children under 2 years old.
The study provides evidence to support the development of Vi-conjugate vaccines as a control measure to reduce the burden of typhoid fever, and the authors say that phase 3/4 and cost-effectiveness studies are now needed. The World Health Organisation's (WHO) Strategic Advisory Group of Experts is due to consider the use of Vi-conjugate vaccines for the control of typhoid fever in October 2017, with subsequent decisions on financing being made by the Global Alliance for Vaccines and Immunisation.
Typhoid affects between 12.5 and 20.6 million people worldwide in regions with inadequate water quality and poor sanitation, particularly in south Asia and sub-Saharan Africa. 1 in 100 cases are deadly and approximately 3% of cases become chronic carriers.
Typhoid is caused by Salmonella enterica serovar Typhi (S. Typhi bacteria) and is usually treated with antibiotics, but antibiotic resistance is increasing. Children are particularly susceptible to typhoid, but no vaccine is licenced for worldwide use in children under 2 years, contributing to poor adoption of typhoid immunisation programmes.
The authors note that because of the design of the study, the participants were not representative of the populations where typhoid is endemic and where Vi-TT might eventually be deployed. Nevertheless, other studies of the vaccine have found that participants as young as 2 months old have a high immune response and that antibody responses can last up to 8 years post-vaccination even in typhoid endemic countries.
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