AstraZeneca's late-stage coronavirus vaccine trial in the U.S. remains on hold as federal investigators seek "answers to important questions" over its safety for patients, Health and Human Services Secretary Alex Azar told CNBC on Wednesday.
Global clinical trials for AstraZeneca's Covid-19 vaccine, called AZD1222, were placed on hold Sept. 6 after one of the participants in the U.K. reported a serious adverse reaction. Following an investigation, AstraZeneca said on Sept. 12 that it had resumed trials in the U.K., though the U.S. trial has since remained on hold.
"Look at the AstraZeneca program, phase three clinical trial, a lot of hope. Single serious adverse event report in the United Kingdom, global shutdown and hold of the clinical trials," Azar told CNBC's "Squawk Box" in defense of the Trump administration's quick development of a Covid-19 vaccine amid safety concerns.
"Still on hold here in the United States as the (U.S. Food and Drug Administration) seeks answers to important questions to verify safety information around those vaccines. Plan is by the book, we're keeping patient safety at the center of everything we do," he said.
AstraZeneca spokeswoman Michele Meixell confirmed that the vaccine's trial in the U.S. remains on hold.
"Regulators in each individual country determine when trials can start and they do this in their own time frame," she said in a statement to CNBC. "On the current U.S. trial status specifically, we are continuing to work with the FDA to facilitate review of the information and the agency will decide when the U.S. trial can resume."
The United States has been racing to develop, manufacture and distribute a coronavirus vaccine in record time as part of the Trump administration's Operation Warp Speed, a partnership between federal agencies, including HHS, the FDA, the U.S. Centers for Disease Control and Prevention, and the Department of Defense.
AstraZeneca launched its late-stage trials at the end of August and is one of at least four vaccine candidates, along with Pfizer's and Moderna's, in late-stage trials. Johnson & Johnson announced Wednesday that it has also started late-stage trials for its vaccine.
However, the fast-tracked timing of the Covid-19 vaccine has left some Americans concerned about its safety. New CNBC/Change Research polls found that most voters worry President Donald Trump is pushing to release a coronavirus vaccine too quickly in order to boost his reelection chances this year. Nationally, only 42% of likely voters said they will definitely or probably receive the inoculation when it first becomes available.
The FDA is expected to announce new and more stringent standards for approving an emergency authorization of a Covid-19 vaccine in an effort to boost transparency and public trust, The Washington Post first reported on Tuesday.
"The president's made it clear, I've made it clear, the FDA commissioner has made it clear, and the CEOs of the relevant companies have made it clear — this is going to be by the book based on science, data and the law," Azar told CNBC.