Washington CNN  — 

Sen. John McCain returned to Arizona Sunday to continue recovering from the side effects of chemotherapy for a brain tumor and will not be in Washington for the tax vote this week, two sources close to McCain confirmed to CNN.

McCain is unlikely to return to Washington this year, one of the sources said. But his ability to get on a plane to Arizona was a “good sign,” a source said.

McCain left Walter Reed Medical Center “exhausted, but ok,” a Republican close to him said.

McCain’s daughter, Meghan McCain, confirmed that the senator would be in Arizona for Christmas.

“Thank you to everyone for their kind words. My father is doing well and we are all looking forward to spending Christmas together in Arizona,” she tweeted Sunday afternoon.

Dr. Mark Gilbert, chief of neuro-oncology at the National Institutes of Health’s (NIH) National Cancer Institute, released a statement Sunday saying McCain “continues to improve” after being treated for a viral infection. He added the senator is also responding positively to his ongoing cancer treatment.

McCain’s office released a statement Sunday night: “Senator McCain has returned to Arizona and will undergo physical therapy and rehabilitation at Mayo Clinic. He is grateful for the excellent care he continues to receive, and appreciates the outpouring of support from people all over the country. He looks forward to returning to Washington in January.”

The Republican senator was admitted into the hospital on Wednesday after missing a third straight day of votes in the Senate. McCain suffers from a type of brain tumor called a glioblastoma. He was diagnosed with the tumor following surgery to remove a blood clot in July.

The final vote on the Republican tax bill is expected to take place early this week.

The passage of the tax bill, however, does not hinge on McCain’s support because the GOP has a 52-48 vote advantage in the Senate. Vice President Mike Pence can also cast a tie-breaking vote, should it come to that.

The White House and Senate Majority Leader Mitch McConnell’s office have been planning for McCain’s potential absence during the tax bill vote, making Tennessee Republican Sen. Bob Corker’s announcement Friday that he would support the plan critical. He did so as an insurance policy for McCain’s absence, two Republicans close to the matter said.

Last week, sources in the Senate told CNN they were worried about McCain recently, following a week when the Arizona Republican was kept from the Senate to address side effects from his brain cancer treatment.

The sources described McCain as looking increasingly frail and said he has not spoken up in recent GOP meetings the way he had before, in addition to his absence this week for treatment at Walter Reed Medical Center.

CNN’s Ashley Killough, Ted Barrett and Phil Mattingly contributed to this report.