Why can't you get tested? They're not going to narc to your parents.

And so what if you test positive? Are you not allowed to "spend time with" your boyfriend? If you're not ready to tell them you're staying together, just be selective with the language you use. It doesn't take that much exposure for a breakthrough case - you're concerned that what, your highly educated in the field of medicine parents are going to jump to the conclusion that the only conceivable way you could have gotten COVID around the same time as your BF would be if you had had some intense 48 hour tantric sex session? If they're telepathic enough to infer you're staying with your BF because you mention you have COVID, they're telepathic enough to infer it even if you don't.

And in any case you sound like old enough to make these sorts of decisions for yourself anyhow if you're not living with your parents anymore.