Vikings wide receiver Percy Harvin remained absent Thursday as the Vikings continued to practice in preparation for Sunday's game at Chicago.

Coach Brad Childress said Wednesday that Harvin was dealing with another migraine and that appears to still be the case. Harvin was not at Winter Park on Wednesday and there has been no sign of him again Thursday. Harvin missed one game last season because of his longtime issues with migraines.

Before Wednesday, Harvin, who is dealing with a sprained left ankle, had not missed a practice because of a migraine since Sept. 22. Harvin was found to have sleep apnea during the preseason and that had helped relieve some of the headaches.

Although the Wednesday and Thursday practices are important, there is a good chance Harvin will play Sunday if he can do anything in practice on Friday. Even if he can't, Harvin still will have a chance to play if he begins to feel better.

Cornerback Asher Allen (concussion) and safety Jamarca Sanford (hamstring) also are not practicing for the second day in a row, although both are on the field observing the session.

Brett Favre, who is dealing with a stress fracture in his left ankle and a fracture in the same heel, sat out practice on Wednesday and is taking part today, probably on a limited basis. It's likely that Favre will be given Wednesdays off for an extended period so he can get a third day of rest. Favre also is bothered by a calf injury that is listed on the injury report.