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 immunization during the second wave of the world’s second-deadliest Covid-19 outbreak.
The letdown after a more promising partial data disclosure last week may also contribute to criticism that vaccines developed by Chinese manufacturers are not subject to the same public scrutiny as U.S. and European alternatives.
Last week, the Brazilian researchers had celebrated results showing 78% efficacy against Covid-19 cases, a rate they have since described as “clinical efficacy.”
Ricardo Palacios, medical director for clinical research at the Butantan biomedical center in Sao Paulo, said the new figure included infections classified as “very mild” because they did not require clinical assistance.
Researchers at Butantan delayed announcement of their results three times, blaming a confidentiality clause in a contract with Sinovac.
In the meantime, Turkish researchers said last month that CoronaVac was 91.25% effective based on an interim analysis. Indonesia gave the vaccine emergency use approval on Monday based on interim data showing it is 65% effective.
Butantan officials said the design of the Brazilian study, focusing on frontline health workers during a severe outbreak in Brazil and including elderly volunteers, made it impossible to compare the results directly with other trials or vaccines.
Still, Covid-19 vaccines in use from Pfizer Inc with partner BioNTech SE and Moderna Inc proved to be about 95% effective in preventing illness in their pivotal late-state trials.
The disappointing CoronaVac data is the latest setback for vaccination efforts in Brazil, where more than 200,000 people have died since the outbreak began – the worst death toll outside the United States.
Brazil’s national immunization program currently relies on CoronaVac and the vaccine developed by Oxford University and AstraZeneca Plc – neither of which has received regulatory approval in Brazil.
Brazilian health regulator Anvisa, which has stipulated an efficacy rate of at least 50% for vaccines in the pandemic, has already pressed Butantan for more details of its study, after it filed for emergency use authorization on Friday.
AstraZeneca failed to deliver active ingredients to Brazil over the weekend, leaving t