It was announced on Tuesday that six players and two members of their coaching staff had the virus but this week’s second round of testing has now been completed.
A total of nine Hull players and three staff members have tested positive.
The remainder of the East Yorkshire club’s players, coaches and backroom staff have returned negative results for a second time.
All the Salford players and staff have tested negative for the second time this week and the club have already announced two further rounds of testing next week.
An RFL statement read: “Hull FC continue to work closely with the Rugby Football League and Public Health England.
“Both the club and Rugby League Cares are providing welfare support to the players and staff affected by Covid-19.
“Following the postponement of this weekend’s scheduled Betfred Super League fixture against Castleford Tigers, Hull FC’s Coral Challenge Cup Sixth Round tie against the same opposition which was scheduled for next Saturday, August 22, has also been postponed in-line with Public Health England self-isolation guidelines.”