B747-400 First Officer reported severe turbulence in cruise flight resulting in temporary loss of aircraft control.

Narrative

Severe turbulence after passing leading to 1500 ft. altitude deviation. Prior to the event; we had been working an issue WXR FAIL; for which the aircraft had also previously been written up for. There was no indication on the radar or from ATC of any significant weather or convective activity. We were in IMC; in what we expected to be a thin layer of stratus clouds. The flight had been smooth with low winds and wind shear number 2 (low) as projected from our flight plan. When the event occurred I was PM and the relief Captain was Pilot flying (PF); with the Captain in observer seat receiving a briefing before taking over command for approach and landing. The aircraft suddenly jolt followed by immediate altitude loss. The aircraft was in zero G and a brief moment of negative G loading; leading to everything in the aircraft not strapped down becoming airborne; including the blue water in our lav. Myself and the relief PIC was strapped in front left and right. The three other crew members onboard became airborne; with two hitting the ceiling and one experiencing headache and some neck pain. The aircraft lost approximately 1500 ft. and momentarily exceeded min and max maneuvering speeds with the stick shaker activating momentarily. The relief PIC who was PF disconnected the autopilot and auto throttles and maintained a constant pitch attitude; only a very minor roll movement was experienced. After the turbulence calmed; a climb back to FL380 was initiated and the event reported to ZZZZ Control and company via ACARS. We proceeded to ZZZZ as planned since it was the closest suitable airport. We alerted company to have medical personnel standing by in ZZZZ in case our potentially injured crew member needed assistance. We elected not to contact med link as we were already starting our descent phase and dedicated our attention to get the aircraft safely on the ground. The flight landed safely in ZZZZ without further incident. The jump seating crew member was examined by medical personnel and their decision was that they were fit to continue to ZZZZ1. After speaking with them; they also indicated that they were fit to continue. A severe turbulence entry was made in the logbook and after inspection; the aircraft and crew continued to ZZZZ1.