I don't think there is screen inside the duct. There is a spring loaded metal flap above the bathroom ceiling that closes when the exhaust fan shuts off. But, I am not sure if it closes tight enough to prevent wasps from entering the house. I would spray wasp killer into the soffit opening to discourage or kill the wasps, or hire a pest control company to handle the problem. As I understand it, wasps do tend to leave after a few months or so.
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