"It's an honor to fill in for him," Vieira said Friday morning on "Today." "You think about the Olympics, and you think the athletes and then Bob Costas."
Costas, who had anchored NBC's coverage of the Winter and Summer Games without interruption since 1988, was sidelined earlier this week by the infection, which left his eyes red and sensitive to bright studio lights and proved distracting for many viewers.
"Bob's eye issue has improved, but he's not quite ready to do the show," NBC Olympics executive producer Jim Bell said Friday.
Current "Today" cohost Matt Lauer stepped in for Costas on Wednesday and Thursday, but said Friday he needed a break from the grueling double schedule. ("Today" is also broadcasting daily from Sochi.)
By stepping in for Costas, Vieira will also be making television history, becoming the first woman to anchor NBC's Olympic coverage solo.