Sources close to Paul tell TMZ ... the KISS lead singer is fully vaccinated and feeling fine despite testing positive. But, nonetheless, the band still had to call it quits on their show at The Pavilion at Star Lake in Burgettstown, PA.

There were rumblings on Twitter Paul was having heart issues -- and that's why the show was canceled -- but we're told that's NOT the case. In fact, Paul got on Twitter to let fans know the heart attack rumors were BS, and he's healthy as an ox. His tweet did not mention his positive test.

KISS is currently on tour and their next scheduled concert is Saturday in North Carolina.

A rep for the group tells us, "More information about show dates will be made available ASAP. Everyone on the entire tour, both band and crew, are fully vaccinated."

The rep continues, "The band and their crew have operated in a bubble independently to safeguard everyone as much as possible at each show and in between shows. The tour also has a COVID safety protocol officer on staff full-time that is ensuring everyone is closely following all CDC guidelines."