Laviai Nielsen is a British sprinter specialising in the 400 metres. She won several medals as a member of the Great Britain 4x400 metres relay teams, including silver at the 2017 World Championships in the women's 4 x 400 metres, and at the 2023 World Championships in the mixed 4 x 400 metres, and bronze in the women's 4 x 400 metres at the 2022 World Championships.

The 4 × 400 metres relay or long relay is an athletics track event in which teams consist of four runners who each complete 400 metres or one lap. It is traditionally the final event of a track meet. At top class events, the first leg and the first bend of the second leg are run in lanes. Start lines are thus staggered over a greater distance than in an individual 400 metres race; the runners then typically move to the inside of the track. The slightly longer 4 × 440 yards relay, on an Imperial distance, was a formerly run British Commonwealth and American event, until metrication was completed in the 1970s.