The American teenager was reportedly showing symptoms of the flu, but her mother hoped she would improve with medicine and rest.
But when Katie failed to get better after four days her mother Colette planned to take her back to the doctor.
But instead, she found Katie lifeless in bed after suffering a cardiac arrest.
Colette believes the fatal episode was brought on by the flu and has urged other parents to make sure their children are vaccinated.
She told NBC: 'If I could do one thing, I would have never even taken her home [from the doctor’s].
'I would have made sure she was at the hospital sooner.
'I didn’t know that it wasn’t just flu that wouldn’t go away. It didn’t and it killed her.'
Colette paid tribute to her much-loved daughter, saying she was ‘so special’ to many people.
A GoFundMe page has been created to support the family.
The fundraising page reads: 'Katie’s sisters and mom have a long road to recovery from this loss and will need all the support they can get as they put the pieces of their life back together.'