Coach Brad Childress confirmed Tuesday that Harvin has had "some" migraines recently. Childress said Harvin is resting at his home in the Twin Cities area.
"I'm not sure when he'll be with the team. If I knew I would tell you," Childress said.
The second-year standout from Florida, who made the Pro Bowl as a rookie kickoff returner, left the Vikings nine days ago when his grandmother died and has not been back to camp.
Harvin has had migraines for most of his life. Last season, the condition kept him out of a December game and out of practice for most of the week before the NFC Championship Game.
"God willing, he is going to work through this," Childress said.
Harvin has seen several specialists and visited the Mayo Clinic in Rochester to seek a cure.
"It's ongoing," Childress said, adding: "The only thing that would be wrong is if we quit trying to find things and throw our hands up in the air."
Wide receiver Sidney Rice has not practiced yet because of a hip injury, further testing the team's depth at the position. The Vikings worked out two free agents and signed wide receiver Freddie Brown, releasing safety Terrell Skinner to make room.
Asked what he likes about Brown, Childress said: "He's fresh. With eight receivers in here we are running the legs off of some of these guys."
Brown was a seventh-round draft pick by Cincinnati in 2009 who spent some time on the practice squad last season. The Bengals cut him last month when they signed Terrell Owens.
The Vikings were forced off the field Tuesday morning by rain and lightning, holding a limited workout in the Minnesota State University fieldhouse before donning full pads for the afternoon workout.
The rain came again, along with some dark, ominous clouds, but the Vikings hustled to finish their practice before it got too stormy. Along with Rice and Harvin, three more offensive starters sat out with injuries, center John Sullivan, right guard Anthony Herrera and tight end Visanthe Shiancoe. Running back Darius Reynaud returned after missing the past week with a sprained left ankle, but backup tight end Jim Kleinsasser was also out.