Health chiefs in East Sussex have attributed the death of a "healthy" four-year-old girl in November last year to swine flu.
The child, who has not been named, died at the Royal Sussex County Hospital in Brighton after contracting H1N1 influenza, NHS Brighton and Hove confirmed.
The girl did not have any underlying health symptoms.
Tom Scanlon, Director of Public Health at NHS Brighton and Hove, said: "Our thoughts are with the child's family and friends, and we would urge everyone to respect their privacy during this difficult and tragic time.
"This is tragic evidence of how serious swine flu can be and why we should not be complacent. Although the majority of people in Sussex who have become unwell with swine flu have only experienced mild symptoms, it can have a much more serious and tragic effect and claim lives.
"Swine flu is present in the community and we would encourage everyone to be aware of its symptoms and what to do if you or a member of your family becomes unwell."
The symptoms of the pandemic include fever or a high temperature, lack of appetite, tiredness, sore throat, coughing, pain in muscles and joints, headache and chills.