The 62-year-old TV presenter revealed she had "finally" contracted coronavirus over two years into the pandemic and was isolating at home, while the former Countdown star stepped in to replace her at the last minute.
Kelly revealed on social media: "The dreaded COVID has finally caught up with me. I am so thankful to the scientists who developed the vaccine!"
She added: "I tested positive at the weekend. Thanks to the vaccine it’s not too bad and I hope to be back at work very soon. Huge thanks to @carolvorderswho will be hosting @lorraine until I’m back."
The TV host revealed she is being looked after by her 27-year-old daughter Rosie.
Vorderman, 61, told viewers: "Sadly Lorraine has caught the dreaded you know what, and I'm sitting in for a few days here. But we are wishing her a speedy recovery."
Vorderman tweeted: "I'll be the Deputy to the Deputy @clbleakley to the Boss Lorraine & hosting @lorraine for a few days this wk [sic]"
Kelly attended Wimbledon Tennis Championship just days before she tested positive for coronavirus, enjoying the luxury of the Centre Court Royal Box and dazzling in a £15 dress from Primark.
Vorderman was the resident mathematician on the long-running Channel 4 game show, before Rachel Riley, between 1982 and 2008.
She said: "I loved my time with Richard [Whiteley] particularly, you know. We laughed and laughed and laughed.
"Everyone who watched, we never ever said the word 'viewer' – they were 'Countdowners'. And it was a family, and it was a very special time and a long time. So I call that old Countdown, previous Countdown."
Watch: Lorraine Kelly urges Carol Vorderman to host Countdown