B777 First Officer reported an 'OUTFLOW VALVE AFT' EICAS message during takeoff roll. Crew performed appropriate checklist and continued to destination.

Narrative

During the takeoff roll as pilot flying (FO) I heard the captain say continue. I did not see any eicas or master caution or master warning light. I looked to the center display to see OUTFLOW VALVE AFT eicas illuminated which was an advisory. I looked back at my airspeed and recall seeing around 100kts. After takeoff upon reaching a safe altitude I took the aircraft and the radios; turned the autopilot on; and the captain ran the non normal checklist. I queried ATC to maintain 5;000 ft as the aircraft was not pressurizing and we could feel it in our ears. I advised ATC we needed to run a checklist that we had a mechanical problem. We stayed in the vicinity of the airport while the checklist was completed and the captain conferred with dispatch to ensure the safest course of action and to continue. Ultimately we were able to control the aft outflow valve in manual mode and everything else worked properly; and we were pressurizing. It was decided we would continue the flight as planned. ATC was notified we finished our procedure and we were recleared on course and issued a climb. During the climb out all the way to cruise no abnormalities were discovered and the aircraft pressurized normally. The flight continued to completion with no more abnormalities noticed.