A coronavirus vaccine developed by China's Sinovac Biotech was just 50.4% effective at preventing symptomatic infections in a Brazilian trial, researchers said on Tuesday, barely enough for regulatory approval and well below the rate announced last week.

The latest results are a major disappointment for Brazil, as the Chinese vaccine is one of two that the federal government has lined up to begin immunisation during the second wave of the world's second-deadliest Covid-19 outbreak.