Date: 202101
Local time of day: 1801-2400
Location: ZZZ.ARTCC, US
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class A ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Light Transport; Low Wing; 2 Turbojet Eng
Reporter function: Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Time Pressure; Situational Awareness; Distraction; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued New Clearance
Contributing factors: Staffing; Environment - Non Weather Related; Human Factors
Primary problem: Environment - Non Weather Related

Narrative: I was working [Sector] X at the time of the incident.  Our area was not staffed properly; so I had been on position for about 2:40 when the event occurred; and was working my third different radar scope of the session.  I had been moved twice due to partially certified controllers kicking me out of my previous position.  I had also just returned from XX days off.The sector next to me (XY) was very busy; and part of my attention was diverted to that scope because there was inadequate staffing to provide them a tracker or D-side.  I know this is not something I should do; but the controller at XY seemed over loaded.I took hand offs on Aircraft X and Aircraft Y at about the same time.  I saw the traffic immediately; and turned Aircraft X to a 270 heading; and issued Aircraft Y 20 degrees left.  I've been certified at ZZZ1 radar for over X years; and have seen this same scenario thousands of times in my career.  I continued doing other things; then looked back on the situation and saw for some reason my 20 degree turn was not going to be enough.  I am not sure if Aircraft Y actually turned 20 degrees; or maybe wind was a factor.  I issued a 10 degree left turn to Aircraft X; confident that this would resolve the issue.  Upon checking back a minute or so later; I saw that this was not going to be the case.  I issued another 20 degree left turn to Aircraft Y and instructed them to expedite the climb.  I stopped Aircraft X at FL190; but did not issue further turn because it would have violated ZZZ2 RAPCON airspace.  About this time both aircraft responded to RA's.  I waited for both to complete the RA maneuvers and cleared them back on course and up to requested altitude.First; I feel that personally I should have used altitude separation instead of; or in addition to; lateral separation.  I should have ensured that the vector was enough before giving up vertical separation.  It is something I have told my trainees hundreds of times in my career; and something that I should have done.Also; I recommend that the area be staffed correctly.  We have been on COVID-19 schedules since March; and there seems to have been a shift in mentality regarding staffing.  As a facility we worked nearly X;X00 tracks yesterday; and my area's night shift only had X CPCs (Last year we would have had 5 or 6 on the shift.)  The result of this was that I was on position for nearly 3 hours without a break; during which my LoSS occurred.  I understand that we are trying to keep the crews isolated from one another; but we also need to staff appropriately for the traffic.  If we cannot staff enough people on a shift; then traffic needs to be slowed down.




Synopsis: Center Controller reported a loss of separation and attributed it to being overworked; and short staffed at the Center.