The number of people in Ashburton who have contracted hepatitis A could be much higher than the 28 cases confirmed so far.
The Canterbury District Health Board has begun vaccinating the town's pre-schoolers to try to prevent the spread of the liver disease that kills one in every 50 people who contract it.
Canterbury Medical Officer of Health Dr Alistair Humphrey told Morning Report nearly all the confirmed cases have been tracked back to a single preschooler who went on holiday overseas with their family.
Public health officials say the case demonstrates how important it is to be inoculated before going on holiday.
