Brazil's health regulator, Anvisa, halted Phase 3 trials of Sinovac Biotech's CoronaVac COVID-19 vaccine candidate on Monday following a serious adverse event in a participant. Sinovac expressed confidence in the vaccine's safety profile on Tuesday.
Over 150 COVID-19 vaccines are in development worldwide, with only a select few advancing to Phase 3 trials—the final stage before regulatory approval. While Pfizer's collaboration with BioNTech has progressed smoothly, Sinovac's effort has hit a snag.
Anvisa suspended the CoronaVac trial "after a serious incident occurred in a patient on October 29." Details remain limited, but the patient's life may have been at risk. Anvisa's statement notes such incidents could involve "death, potentially life-threatening side effects," or "severe disability."
The agency will now "review data to date and assess the benefit-risk balance for resuming the study." No new participants will receive doses during this pause. Sinovac reaffirmed its trust in the vaccine's safety despite the interruption.
Such pauses are routine in large-scale trials to investigate any unexplained adverse events and protect participant safety. In September, AstraZeneca and the University of Oxford paused their Phase 3 trial after a participant's adverse reaction.
Biostatistician Adrian Esterman from the University of South Australia downplayed concerns at the time, telling the Sydney Morning Herald: "It's extremely common. Participants sometimes experience death or stroke in trials." He added, "Research isn't linear—hiccups and setbacks are part of the process."
Investigations cleared AstraZeneca's vaccine, allowing trials to resume soon after in the UK, followed by South Africa, Brazil, Japan, and the US. The study continues today.
