Those countries are on the Government "red list", meaning Harding would have to isolate for 10 days on his return, ruling him out of matches against Fleetwood Town and Bolton Wanderers.
The Premier League and Football League clubs have made a collective decision not to release players due to travel to red list countires, in defiance of FIFA rules. Tottenham Hotspur and Aston Villa have reluctantly broken ranks.
Warne has not joined the calls from some in football for players to be exempted from quarantine on their return.
"I can't see why footballers can be exempt (from quarantine)," he said recently. "If a load of footballers are exempt and bring Covid into certain areas you're talking about people losing their lives so I don't think it's really it's up to debate.
"Sport plays a massive part in society but the health of the world has to come first."
As it is, the only Millers game Harding will miss is the September 7 Football League Trophy game at Doncaster Rovers, which he most likely would have sat out anyway.
Rotherham's weekend match at Lincoln City has been postponed because of international call-ups.
Millers has played every minute of the club season so far on the left of a back three.