A Medical Officer of Health says it is still unclear where a Christchurch child contracted measles last week.
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The four-year-old, who was diagnosed with a mild case of the disease last week, had its first vaccine at 15 months but not the second shot at four years.
Children who were not immunised were asked to stay away from the two preschools the child attends.
Canterbury Medical Officer of Health Daniel Williams said the Canterbury District Health Board was working hard to trace the source of the disease.
Dr Williams said the vaccine was highly effective, which was why it was important to maintain high levels of vaccinations in the community.