A trial vaccine against typhoid, a disease that kills more than 200,000 people every year, is safe for use, researchers said on Friday.

Aimed at children under two, who are disproportionately affected, the vaccine may prevent more than half of all typhoid infections, they wrote in The Lancet medical journal.

Dubbed Vi-TT, the vaccine studied in the trial is already licensed for use in infants in India, but nowhere else, nor are there any other vaccines for children under two.

Typhoid is caused by an infection with the bacteria salmonella Typhi, usually through contaminated water, particularly in parts of south Asia and sub-Saharan Africa with inadequate sanitation. About one in 100 cases are deadly.

According to the World Heath Organization, typhoid affects about 21 million people per year, and kills around 222,000. Symptoms include fever, headache, nausea, loss of appetite, constipation and sometimes diarrhea.

The vaccine was tested on adult volunteers in Britain, who were given a dose of bacteria. Those who fell ill were treated with antibiotics.

Symptoms were "less severe" in people given the trial vaccine, who also had lower bacterial numbers in their blood, the team found. Those not given Vi-TT received other existing vaccines to enable researchers to compare the results.

The trial, funded by the Bill & Melinda Gates Foundation and the European Commission, continues.