She won two bronze medals at the 2024 Summer Olympics as part of both British 4 x 400 metres relay teams, mixed and women categories.

Her twin sister Lina had been diagnosed with the same condition some years earlier, and as it has genetic properties, Laviai was tested for the illness.

[8] Nielsen was initially a middle distance runner,[5] but in 2013 she and her twin sister were approached at an event and persuaded to change specialism to the 400 metres.

[11] This success earned Nielsen a place on the Jaguar Land Rover Academy of Sport programme, which offers financial support and mentoring.

[13] In the summer, she reached the final of the 400 metres at the 2017 European Indoor Championships in Belgrade, finishing fourth to miss out on a bronze medal by just 0.27 seconds.

[14] On the final day of the championships, she ran the anchor leg of the 4 x 400 meters relay, winning the silver medal behind a strong Polish team.

[24] After winning the 400 metres silver medal at the 2024 British Athletics Championships, Nielsen was subsequently named in the Great Britain team for the 2024 Summer Olympics.

[25] Nielsen ran a big personal best at the London Athletics Meet on 20 July 2024, clocking 49.87s and breaking the 50-second barrier for the first time.