Sinistra, an unvaccinated COVID-19 denier who reportedly once called the disease a "little virus," voluntarily discharged himself from the hospital and opted to treat his sickness at his home in Liege with oxygen.

He believed his relatively young age and physical condition would be enough to protect himself from the COVID-19 symptoms, according to a report by Vice.

"I was born premature and I will continue to fight to the death like a man without ever giving up and dying without regrets," Sinistra said in another social media post before leaving the hospital.

However, Sinistra died at around 10 a.m. on Dec. 15. He is survived by his partner and two children.

Fabian Pavone, Sinistra's former coach, described the kickboxer as a "monument" and "force of nature with a heart of gold" whose death could have been avoided.

"He was a golden guy. I will never forget him. He was the strongest in Belgium in the heavyweight category," Jordan Sferrazza, one of Sinistra's friends, said.

A little over 90% of Belgium's adult population has received two doses of the COVID-19 vaccine, Reuters said, citing data provided by the country's Sciensano national public health institute.

Thousands of protesters marched on the nation's capital of Brussels for a third time on Sunday after new COVID-19 restrictions were imposed by the government to counter a spike in infections as the omicron variant of the virus spread across Europe.

Belgium has reported a total of 2,017,447 COVID-19 infections and 28,035 deaths since the pandemic began, data from Johns Hopkins University showed.