Here is a story from CNN via the Mercury News about 3 teachers sharing a classroom. Anecdotal, but makes me think these are signs of a lack of immunity. A positive, negative, positive test.
> A month after they caught the virus that killed their colleague, Martinez and Skillings are still struggling.
> Martinez says fatigue is lingering, she still has a cough, and she continues to take breathing treatments to relieve tightness in her chest. She recently tested negative and retested.
> Skillings says she thought she was getting better, but recently her cough returned full force. She tested a week ago and it came back positive again.
https://www.mercurynews.com/2020/07/12/three-arizona-teacher...