For the fourth quarter, we estimate that the incremental mortality due to COVID was approximately 1,100, compared to approximately 1,600 during the third quarter.
Transplant is a preferred treatment option for most of our patients and during 2021, despite the challenges posed by the COVID pandemic, we celebrated that nearly 8,000 DaVita patients received a transplant, exceeding our pre-pandemic level.
I've been fortunate enough to be part of DaVita Village for over 20 years and in all that time across my many roles, I've never experienced the labor market as challenging as we face today.
These programs added approximately 12,000 ESKD patients and an additional 12,000 CKD patients across 11 value-based programs in different markets.
In light of our upfront cost of these programs and the lag of shared savings payment, as we discussed in November, we continue to expect that our operating loss in 2022 in our U.S. ancillary segment will increase by approximately $50 million, although this could increase or decrease depending on the number of new arrangements we enter into during the year.
This resulted in a full-year adjusted operating income increase of approximately 3% over 2020.
Adjusted earnings per share from continuing operations grew by approximately 26% year over year, and we generated more than $1.1 billion of free cash flow, which we largely deployed to return capital to our shareholders.
For 2022, we expect adjusted operating income guidance of $1.525 billion to $1.675 billion.
The midpoint of this guidance range is $35 million below our expectations from Capital Markets Day last November, which is primarily driven by our updated views on COVID and labor costs.
We still believe we can deliver the long-term compounded annual growth of adjusted operating income of 3% to 7% that we discussed at Capital Markets Day.
As Javier mentioned, our fourth-quarter results were slightly above the midpoint of our revised guidance.
Q4 results included a net COVID headwind of approximately $80 million, an increase relative to the quarterly impact that we experienced in the first three quarters of the year primarily due to the impact of the incremental mortality from the delta surge in Q3 and some temporary labor cost increases.
For the year, we experienced a net COVID headwind of approximately $200 million.
As Javier said, the incremental mortality due to COVID in the fourth quarter was approximately 1,100, compared to approximately 1,600 in Q3.
U.S. dialysis treatments per day were down 135 or 0.1% in Q4 compared to Q3.
U.S. dialysis patient care cost per treatment were up approximately $6 quarter over quarter, primarily due to the increased wage rates and health benefit expenses.
Our adjusted effective tax rate attributable to DaVita was 16% for the fourth quarter and approximately 22% for the full year.
Finally, in 2021, we repurchased 13.9 million shares of our stock, reducing our shares outstanding by 11.5% during the year.
We have repurchased to date an additional 1.4 million shares in 2022.
Now looking ahead to 2022, our adjusted OI guidance is a range of $1.525 billion to $1.675 billion, and our adjusted earnings per share guidance is $7.50 to $8.50 per share.
The midpoint of the OI guidance range is $37 million below the $1.635 billion that we discussed during our recent Capital Markets Day due to offsetting puts and takes.
At the midpoint of our guidance range, we have incorporated the following assumptions related to COVID: excess patient mortality due to COVID of 6,000.
This, along with our normal growth drivers, would result in a total treatment growth range of approximately 1.5% to 1%.
Our guidance assumes an incremental increase of between $100 million and $125 million in labor costs above a typical year's increase, which is $50 million higher than what we communicated at Capital Markets Day.
Third, we anticipate a year-over-year incremental operating loss in the range of $50 million as we continue to invest to grow our IKC business; and fourth, we will also begin to depreciate our new clinical IT platform, which we expect to be approximately $35 million in 2022 and will begin in Q2.
We are forecasting our tax rate at 25% to 27% due to nondeductibility of valid expense.
Looking past 2022, we continue to expect compounded annual OI growth relative to 2021 of 3% to 7% and compounded annual adjusted earnings per share growth relative to 2021 of 8% to 14%.
Finally, we expect free cash flow of $850 million to $1.1 billion in 2022.

Summary:
As Javier mentioned, our fourth-quarter results were slightly above the midpoint of our revised guidance.
We have repurchased to date an additional 1.4 million shares in 2022.
Now looking ahead to 2022, our adjusted OI guidance is a range of $1.525 billion to $1.675 billion, and our adjusted earnings per share guidance is $7.50 to $8.50 per share.
Finally, we expect free cash flow of $850 million to $1.1 billion in 2022.