She was from that day left unable to move or talk and within a year became the 13th person globally to be diagnosed with an incurable variant of acute disseminated encephalomyelitis, or Adem. It left her physically trapped but cognitively unimpaired.
Grace was the first Kiwi to have contracted the disease, which most likely struck in the wake of the throat infection doctors say, and her specific condition was the worst a specialist had encountered.
Her father said yesterday the recovery of her ability to vocalise was a miracle.
"I'll admit it is miraculous. It's very hard to explain. What's miraculous about it is that for four years we've tried to get her to talk. She was the talker in the family. We called her 'Lady Blah Blah'," Mr Yeats said.
"For nearly four years we have tried and tried and tried to get her to communicate with us because it's just so important, especially in a situation like this. We need Grace to be able to say yes or no."
The former St Mary's School pupil had before waking from surgery only ever been able to indicate "yes" with a slight movement of her index finger, Mr Yeats said, and had no similar signal for no.
"So we've gone from that, despite constant coaching from us, to vocalisation," he said.
"My hope is that if that brain connection is made, if the connection is there, then there's hope. To be quite honest, I'd lost hope a long time ago. Tracy gets the credit for persisting and believing."
Mr Yeats said Grace's brothers Finn and Angus were like himself "very pleased" at her recovery, which had been captured in videos posted to her Facebook page where she says "yes" and "no", repeats the alphabet, and says thank you before cheering.
Grace's mum, who could not be reached yesterday, had heralded the good news on Facebook with an exuberant post confirming that Grace had also responded "South Africa" when asked about her brother Angus, and replied Cake by the Ocean when quizzed on the song that was playing in her hospital room.
"I always hoped I could say this at some point," her mum said in her post.
"Grace has woken from a very long deep sleep and she can talk. Yes, you heard that right. She is talking," she said.
"Please God, let this be a big breakthrough that doesn't go away. Please let it stay."
Mr Yeats was last with Grace at Wairarapa Hospital, when she was desperately ill, before she was airlifted to Auckland. He will fully accept her astonishing recovery, he said, only "when I see her again, when I'm with her again".
"She was very, very sick last week. That's the other remarkable thing. She's gone from a life or death situation, literally. When I saw her last in Wairarapa Hospital, she was scarcely well enough to fly.
"It is amazing. That's nearly four years of being completely mute and not being able to form a word and now she's vocalising. I just hope those connections in the brain that allow her to do that, will allow her to do other things and say more, you know."
Grace had been transferred out of the high dependency unit at Starship Hospital after surgery, he said, although he was unsure when she will return home.
