Brazil's decision to halt trials of a Chinese-developed Covid-19 vaccine triggered a politically charged row Tuesday as a top health official expressed "indignation" and far-right President Jair Bolsonaro claimed the ruling as a personal victory.

Brazilian health regulator Anvisa announced late Monday it was suspending clinical trials of CoronaVac, developed by Chinese pharmaceutical firm Sinovac Biotech, after a "severe adverse incident" involving a volunteer in the study.

However, the public health center coordinating the trials in Brazil, the Butantan Institute, said there was no connection between the incident and the vaccine.

Privacy regulations prevent public health officials from disclosing details on the incident, which came as a setback for one of the most promising vaccines aiming to end the pandemic.

But the coroner's office that carried out an autopsy on the deceased volunteer's body told AFP police were investigating the death as a suicide.

"There is no relation with the vaccine," said Dimas Covas, head of the Butantan Institute, a respected public health center.

Anvisa's decision, he told a news conference, "came as a surprise... and caused indignation here at Butantan."

"We don't even know if this patient took the vaccine or a placebo," said the coordinator of the Sao Paulo state government's Covid-19 task force, Joao Gabbardo.

Sinovac also said the incident was unrelated to CoronaVac, adding it was "confident in the safety of the vaccine."

Anvisa chief Antonio Barra Torres told a separate news conference the agency had made a "technical decision" based on "incomplete and insufficient data" from the Butantan Institute on the cause of the incident.

CoronaVac has been caught up in a messy political battle in Brazil, where its most visible backer has been Sao Paulo Governor Joao Doria, a leading Bolsonaro opponent.

The president has thrown his support behind another vaccine, developed by Oxford University in Britain and the British-Swedish pharmaceutical firm AstraZeneca.

Dimas Covas, director of the Butantan Institute, speaks during a press conference in Sao Paulo on November 10, 2020
Dimas Covas, director of the Butantan Institute, speaks during a press conference in Sao Paulo on November 10, 2020 AFP / Nelson ALMEIDA

Bolsonaro and Doria have also sparred over whether vaccination should be mandatory.

The president, who has railed against CoronaVac as "Joao Doria's Chinese vaccine" and blocked the federal government from purchasing it, claimed the regulatory decision as vindication.

"Death, disability, malfunction. This is the vaccine Doria wants to require all Sao Paulo residents to receive," Bolsonaro wrote on Facebook.

"The president said this vaccine could never be mandatory. Chalk up another win for Jair Bolsonaro," he added.

Sao Paulo state has a deal with Sinovac to buy 46 million doses of CoronaVac -- six million produced in China and the rest produced in Sao Paulo, which broke ground last week on a factory to make the vaccine domestically.

The Sinovac and Oxford vaccines are both in Phase 3 trials, the final stage before regulatory approval.

Both are being tested in Brazil, the country with the second-highest death toll in the pandemic after the United States, with more than 162,000 people killed by the new coronavirus.

Brazil is also helping carry out Phase 3 testing of the vaccine developed by US pharmaceutical giant Pfizer and Germany's BioNTech.

Pfizer said Monday its vaccine had shown 90 percent effectiveness, unleashing a surge of optimism worldwide that an end to the pandemic could be within reach.

Bolsonaro faces criticism from experts for his handling of the pandemic, which has included playing down the virus, opposing lockdown measures and relentlessly promoting the drug hydroxychloroquine despite studies showing it is ineffective against Covid-19.

Opponents sharply criticized what they called his politicized handling of Monday's decision on CoronaVac.

"Prison is too light a punishment for these scums who play politics with vaccines, which are the only way out of the biggest public health and socioeconomic crisis in history," tweeted center-left politician Ciro Gomes, who ran against Bolsonaro in the 2018 presidential election.