Brazil's health regulator has said that the death of a 16-year-old who had a first dose of the Pfizer (NYSE:PFE)/BioNTech (Nasdaq:BNTX) COVID-19 vaccine was due to a prior blood clot condition not related to the vaccine, Reuters news agency reported on Thursday.

According to Anvisa, the death was caused by an auto-immune disease suffered by the adolescent.

"It was a characteristic clinical picture of thrombotic thrombocytopenic purpura, an autoimmune disease," Anvisa stated.

Thrombotic thrombocytopenic purpura is a rare disorder that causes blood clots to form in small blood vessels throughout the body that can cause serious medical problems if they restrict blood flow to organs such as the brain, kidneys or heart.

The Pfizer COVID-19 vaccine is the only one approved for minors in Brazil.

Brazil's federal government sought last week to halt COVID-19 vaccinations for adolescents while the death was under investigation as a suspected adverse event.