Is 'is' needed in a sentence like this?

Take the sentence below I just found in a book for example:

- And the more unusual the requirement, the less likely we are to find such a person.

I wonder if verb to be is needed after 'requirement', to read:

- And the more unusual the requirement is, the less likely we are to find such a person.

Is verb to be optional and omissible in this kind of sentence?

Thanks in advance.