Kristin Chenoweth commented: "Jimmy please rest. Eat. Repeat. Feel better."
Fallon's news comes after it was revealed fellow talk show host Whoopi Goldberg also recently tested positive for Covid, and is experiencing "very, very mild" symptoms.
Goldberg's The View co-host Joy Behar, 70 said on the show: "They say there's no place like home for the holidays and that's exactly where we are this year.
"Happy New Year from all of our living rooms. As you can see we're back in boxes and doing the show remotely - hopefully for just a week, I'm praying that it's just a week but you never know because this Omicron thing is all over the place.
"Why am I here instead of Whoopi? Well, Whoopi, unfortunately, tested positive over the break but she'll be back probably next week. Since she's vaxxed and boosted, her symptoms have been very, very, mild. But we're being super cautious here at The View."