Sources with direct knowledge tell TMZ ... the incarcerated singer's as COVID-safe as can be, after recently receiving shot #2 from the Bureau of Prisons at MCC Chicago, where's he's currently housed while awaiting trial.

We're told Kelly got his first dose in January and the second this month, so he's now fully inoculated.

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According to the BOP ... 63 inmates at MCC Chicago have been fully vaccinated so far, along with 128 staff members.

As we reported ... Kelly tried twice,and failed twice, to get released from lockup during the early days of the coronavirus pandemic due to the risk of contracting the disease.

STARS GETTING VACCINATEDLAUNCH GALLERY

He doesn't have to worry nearly as much about that anymore with a roughly 95 percent efficacy rate with the vaccines ... but his trial still looms. It's currently scheduled for September.