Brazil's health regulatory agency (Anvisa) on Monday opposed the use of the Russian-made Sputnik V vaccine citing insufficient data.
"Due to the lack of consistent and reliable data, Anvisa's Board of directors unanimously decided not to recommend the exceptional importation of the Russian Sputnik V vaccine," it said in a statement.
Fourteen of the country's states have already imported doses of the vaccines.
Anvisa said it had no or insufficient data on the vaccine's "quality control, safety and efficacy" and that an inspection mission sent to Russia was also unable to "identify the manufacturers of the biological pharmaceutical input used in the production of the Russian vaccine."
"Nor were any manufacturing conditions identified that demonstrate that the products are consistently produced and controlled," it went on, also flagging that its inspection team "was denied access to the facilities of Gamaleya, the institute that developed the vaccine."
"We will never allow millions of Brazilians to be exposed to products without proper verification of quality, safety and efficacy or, at the very least, given the serious situation we are going through, that there is a favourable risk-benefit ratio," Anvisa President Antonio Barra Torres said.
Brazil has recorded nearly 392,000 COVID-19 death since the beginning of the pandemic — the world's second-highest death toll after the United States. More than 14.3 million people have also contracted the virus, according to a tally by Johns Hopkins University.
Sputnik V rejected the decision, writing on Twitter that the unfavourable opinion is "unfortunately, of a political nature" and has "nothing to do with access to information or science".
"The United States Department of Health, in its 2020 annual report several months ago, publicly stated that the United States health attaché 'persuaded Brazil to reject the Russian COVID-19 vaccine'," it added.
According to the manufacturer, the vaccine has now been distributed to 60 countries worldwide, including European Union member states, Hungary and Slovakia.
However, the Slovakian health watchdog also emitted concerns over insufficient data earlier this month, saying the vaccines delivered to the country were different to elsewhere. Sputnik V branded the regulator's findings "fake news" and demanded the country returns the doses it had received.