I have a question about this sentence. I'm trying to help a student understand the grammar, but I'm not sure how to explain it. He wants to use "are described"(because it's passive) in the blank space although the correct answer is "described".

"In one study, 43% of employees ____ as being under heavy stress had weak concentration and poor work performance. "

Any help will be deeply appreciated!

A student wants to use "are described"  (because it's passive) in the blank space although the correct answer is "described". How can I explain the grammar of why it is incorrect?

In one study, 43% of employees ____ as being under heavy stress had weak concentration and poor work performance.