The idea being that...as with Covid itself... people with co-morbidities (like propensity for blood clots etc...) may also be vulnerable to adverse response to certain vaccines. And this needs to be studied more.
Also there is another thread on FR about those who have had Covid recently or already have it but are unaware. They should ideally be screened before getting vaccine. It’s better to wait.
I’ve been getting regular screenings for COVID-19 due to work issues (everyone in a client’s office came down with it in a time period where I’d spent time in the office and could have been infected and another client that said she’d had it and was safe to be around - who turned out to either *not* have had it when she thought or got reinfected. Fun!) Not an issue in my case.