A new typhoid vaccine developed by privately-held Bharat Biotech proved safe and highly immunogenic in a study and could be used to prevent millions of infections if it succeeds in final-stage clinical trials, researchers said on Friday.

Typhoid fever affects between 12 and 20 million people worldwide in regions where the quality of water and sanitation is low, particularly in south Asia and sub-Saharan Africa.

Around 1 in 100 cases is deadly, and about 3 percent of those infected become chronic carriers of the disease.

Results of a mid-stage trial of the vaccine, a so-called Vi-conjugate shot which, its developers say, could also be used safely in babies, showed it was able to prevent half of those vaccinated from developing typhoid when they were exposed to it.

"Our study provides further evidence to support the development of Vi-conjugate vaccines as a control measure to reduce the burden of typhoid fever," said Andrew Pollard of Oxford University's vaccine group, who co-led the trial.

Typhoid is caused by a bacteria called Salmonella enterica serovar Typhi and is usually treated with antibiotics. But access to antibiotics is in poorer regions is sometimes limited, and the bug's resistance to them is on the rise.

Experts say that while children are particularly susceptible to typhoid, no vaccine has yet been licensed for worldwide use in babies under 24 months old.

This study, published in The Lancet medical journal, was conducted in 112 adult volunteers and used a "controlled human infection model". Volunteers are randomly assigned to be given either the experimental vaccine or a control one, and then deliberately exposed to the pathogen. Participants are closely monitored and treated for infection afterwards.

Studies like this have been used in the development of various vaccines, including shots against cholera, since they are a fast and clear way to assess whether a vaccine works.

Nicholas Feasted, an expert at the Liverpool School of Tropical Medicine in Britain, and Myron Levine of the University of Maryland in the United States, said in The Lancet that the trial's results had been "awaited with much anticipation" by global health experts keen to tackle typhoid in endemic areas.

The World Health Organization's expert panel on vaccines is due next month to consider whether to recommend Vi-conjugate vaccines to prevent typhoid. Once the WHO's recommendations have been made, the Global Alliance for Vaccines and Immunisation, which helps fund vaccines at lower prices for poor countries, will decide whether or not it can finance the shots.