"I'm devastated to say that due to early signs and symptoms of Covid and an inconclusive test result.
I'm not allowed to perform today's shows," Michele, 36, wrote on her Instagram Story Saturday.
"I will be testing again and will know more about tomorrow's performance soon," she added.
Julie is going to crush it today as Fanny - as are all of our amazing understudies who have stepped up.
So incredibly this week while we battle a very intense Covid outbreak in our theater."
The actress took over the role of Fanny Brice in the revival Tuesday.
She is expected to return to the show on Sept. 20, according to the musical's Twitter feed.
Julie Benko will play the role until then. Tovah Feldshuh, Ramin Karimloo and Jared Grimes co-star.
The revival opened in late April to mixed reviews with Beanie Feldstein playing Fanny.
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