It wasn't all that long ago that many people saw AstraZeneca (NASDAQ:AZN) as the leader in the race to develop a coronavirus vaccine. The U.K.-based drugmaker partnered with the University of Oxford and got off to a fast start.

AstraZeneca has fallen behind over the last four weeks -- at least in the U.S. Early last month, the company announced that it was temporarily pausing its late-stage clinical studies evaluating COVID-19 vaccine candidate AZD1222. AstraZeneca has since resumed its trials in multiple countries; indeed, AZD1222 is already being reviewed for potential regulatory approvals in Canada and Europe. 

But the chances of a regulatory review in the U.S. for AstraZeneca's coronavirus vaccine this year appear to be dwindling. Why is this one-time Operation Warp Speed favorite progressing so slowly in the U.S.?

A scientist holds a small planet Earth in one hand and injects it with a hypodermic needle.

Image source: Getty Images.

A short pause elsewhere

AstraZeneca initially paused its late-stage studies of AZD1222 on Sept. 6, 2020, because one participant in the U.K. clinical trial developed an unexplained illness after receiving the experimental vaccine. The company referred to the voluntary pause as a "routine action" that would ensure the integrity of the clinical study as an independent committee investigated the matter.

From the outset, AstraZeneca emphasized that some illnesses will occur by chance in large clinical studies. That's exactly what U.K. regulatory authorities decided happened in this case. On Sept. 12, the Medicines and Healthcare Products Regulatory Agency determined that it was safe to resume the clinical trial of AZD1222 in the U.K. 

Over the next few weeks, other countries followed suit. Regulators in Brazil, India, South Africa, and (most recently) Japan, reviewed AstraZeneca's findings and allowed late-stage clinical trials evaluating AZD1222 to resume. 

Health Canada and the European Medicines Agency have begun what's called a rolling review of AZD1222. This process allows AstraZeneca to begin filing portions of the data needed for the review process over time, while clinical trials are still underway. This approach speeds up the regulatory review process and could lead to faster approvals for AZD1222 if all goes well. 

A longer delay in the U.S.

Meanwhile, the U.S. Food and Drug Administration (FDA) still hasn't allowed AstraZeneca's U.S. clinical trial evaluating AZD1222 to restart. AstraZeneca stated in a recent press release that it's working with the FDA to provide all the information needed for the agency to make a decision about resuming the U.S. trial. 

Reuters reported that the FDA has widened its investigation into AZD1222. The news organization said that the FDA is looking at data from clinical studies of similar vaccines developed by the same scientists that worked on AstraZeneca's COVID-19 vaccine. 

The apparent purpose of this wider investigation is to determine if similar side effects that occurred in the U.K. clinical trial of AZD1222 have occurred in other vaccines developed by the University of Oxford team. It's not known how long it will take for the FDA to complete its expanded review.

Could politics be a factor in the FDA's relative sluggishness compared to other countries' regulatory agencies? 

The FDA has come under fire for allegedly moving too quickly to grant emergency use authorization to some experimental COVID-19 treatments, particularly for hydroxychloroquine (approval for which the FDA subsequently rescinded) and convalescent plasma. There has also been speculation that the agency is under pressure to rush EUA for a coronavirus vaccine prior to the U.S. presidential election.

However, Reuters' sources, who are familiar with the details of the FDA's expanded investigation for AZD1222, only said that the agency was "being thorough."

A lot at stake

In May, the U.S. government agreed to fork over up to $1.2 billion for the drugmaker to supply 300 million doses of AZD1222. AstraZeneca can't ship any of those doses until it wins FDA emergency use authorization -- and it can't do that until its late-stage U.S. clinical trial resumes.

An FDA decision to let U.S. testing of AZD1222 proceed could boost the big pharma stock. However, a more important victory for AstraZeneca would be securing emergency use authorization for the COVID-19 vaccine. That's still a real possibility, but AstraZeneca isn't likely to be the first company to win.