AstraZeneca Plc has begun a large-scale human trial of its coronavirus vaccine in the U.S., the company said in a statement, with plans to enroll as many as 30,000 adults to test the shot.

The AstraZeneca shot, invented by researchers at the University of Oxford, is one of the farthest along of numerous COVID-19 vaccines in development. In addition to the U.S. trial, a final-stage test of the inoculation is underway in the U.K. and could yield preliminary results as soon as next month. Other companies that have COVID-19 vaccines in phase 3 trials include Moderna Inc. and Pfizer Inc.

The trial had been delayed for several days, according to researchers from the University of Wisconsin School of Medicine and Public Health who are helping conduct the tests. The university is one of the sites where the trial is being run, and plans to begin injecting healthy volunteers on Tuesday, according to William Hartman, a UW Health anesthesiologist and investigator on the trial. It plans to ramp up to injecting 50 people a day shortly after the Sept. 7 Labor Day holiday, he said in an interview.

An Aug. 27 article in the Palm Beach Post reported that the U.S. trial of AstraZeneca vaccine has been put on hold due to political pressure, perhaps because U.S. regulators planned to grant an emergency use authorization on the basis of the trial in Europe.

Hartman said he was told the hold was due to unspecified operational issues.

“There was a hold on enrollment. We don’t really know what the reason for that was. We were told it had nothing to do with safety and didn’t have anything to do with an EUA, it was just an operational issue,” Hartman said in a phone interview. “On Friday evening, they lifted that hold.”

An email shared with Bloomberg that Hartman received on Aug. 28 from AstraZeneca and contract research organization Iqvia Holdings Inc. stated the hold “was lifted and we have our first patient screened and randomized.”

Americans should understand that “there are no corners that have been cut,” said Hartman, in a phone interview. “We are going to proceed with the trial independent of any political pressure.”

People in the trial will get either the experimental vaccine or a placebo made of saline.

In a separate statement, AstraZeneca CEO Pascal Soriot nodded at concerns that review of a vaccine could face political pressure.

“In recent weeks we have seen an increasing number of questions around the safety and availability of vaccines to fight this terrible COVID-19 pandemic and I want to reiterate my commitment that we are putting science and the interest of society at the heart of our work,” Soriot said. “We are moving quickly but without cutting corners.”