Authorities in Belgium said the child was by far the youngest among the country’s more than 700 victims.
National crisis center spokesman Emmanuel Andre said it is “an emotionally difficult moment because it involves a child, and it has also upset the medical and scientific community”.
He added: “We are thinking of her family and friends. It is an event that is very rare, but one which upsets us greatly.”
It is not yet known if the girl had underlying health conditions.
Mr. Andre said 98 people had died with the infection over the last 24 hours, bringing the total to 705 in a country of around 11.5 million people.
More than 12,705 infections have been confirmed so far in Belgium.