Health regulators abroad have begun giving the OK for AstraZeneca to resume its phase 3 COVID-19 vaccine trial following a single unexplained patient illness, but the FDA has still not cleared the trial to start back up, with FDA Commissioner Stephen Hahn saying the agency will engage in “very significant work” with the company to unravel the safety issue.

Hahn said during an interview with Sen. Tim Scott (R-S.C.) that the FDA will be devoting serious effort to helping the British drugmaker conclude “if there’s a significant safety issue or not.” The agency’s concern revolves around a serious adverse event in a UK patient enrolled in AstraZeneca’s late-stage vaccine trial, the cause of which has not yet been determined.

“We don’t have all the facts, so we don’t know the causation per se of this, but we really need to look into it,” Hahn said. “What FDA does is we take our responsibilities around safety very seriously. We need to take it all seriously no matter where it occurred.”

AstraZeneca said last week that it had nothing new to share on the trial pause in the U.S. and cannot provide details on its discussions with regulators, including the FDA.

Hahn reiterated in his remarks that the agency will only approve or authorize a COVID-19 vaccine if it is found safe and effective through proper research, addressing concerns that the agency could give in to pressure from the Trump administration to deliver a vaccine before the presidential election.

The Drugs Controller General of India (DCGI) is the latest health regulator to allow research on the promising vaccine candidate to resume, giving the Serum Institute of India the go-ahead Tuesday to start its trial back up. UK, Brazilian and South African trials have also begun again.

The FDA declined to comment further on its investigation into the vaccine safety issue. The National Institutes of Health has started its own investigation, according to an agency investigator, though it’s still in the early stages. The medical research agency has expressed great concern about the situation and urged AZ to share more details about the adverse event.