Typhoid, a water-borne and food-borne illness which causes high fever and is estimated to kill 600,000 people a year, has traditionally been thought to be rare among very young children.

As a result, anti-typhoid vaccines have either not been tested on them or are unsuitable for them.

Researchers, led by Professor Maharaj Bhan, conducted a study in Delhi which revealed that typhoid incidence was more common among this age group than thought.

In a study of more than 8,000 people, they found that 63 had typhoid.

And 44% of these were children under five - higher than any other age group.

They also found that severity and duration of fever and the likelihood that the disease required hospital admission was similar for under fives as for adults.

They surmise that previous hospital tests have under-reported the problem, possibly because it is difficult to collect the required amount of blood in preschool children or because the disease presents in a different or mild way in the very young.

Mass immunisation

Writing in The Lancet, they conclude: "Typhoid is a common and significant cause of morbidity between one and five years of age.

"The optimum age of typhoid immunisation and the choice of vaccines needs to be reassessed."

The researchers say two moderately effective vaccines are currently available against typhoid, but neither is being used on a mass scale in developing countries.

They state that one, Ty21a, has not been tested on very young children and is not suitable for widespread use for young children because of its cost and because it requires three separate doses.

The other vaccine, Vi polysaccharide, could be viable, but needs to be adapted for infants and toddlers.

"Our findings support the need for development of new vaccines that may be effective when given in late infancy," they say.

They also call for more research on the incidence of typhoid and age distribution in different parts of the developing world so that cost effectiveness and rational public health policies regarding immunisation can be worked out.