When the World Health Organization (WHO) says that a given anti-coronavirus product is a leader, that's about as official as it gets in the fast-moving world of COVID-19 therapies and vaccines.

Earlier this year, a top WHO executive stated that Gilead Sciences' remdesivir was the leading therapy targeting COVID-19. Remdesivir, of course, has now become the de facto standard of care for the novel coronavirus disease.

Last week, WHO chief scientist Soumya Swaminathan stated in a news conference that one experimental COVID-19 vaccine appears to be "probably the leading candidate." Which vaccine is it?

Vials labeled COVID-19 Vaccine lying on a pile of $100 bills

Image source: Getty Images.

WHO's who

Swaminathan thinks that the COVID-19 vaccine being developed by AstraZeneca (AZN 0.28%) and the University of Oxford should be viewed as the top contender right now. There are two main reasons why.

First, the AstraZeneca-Oxford AZD1222 vaccine is already in a phase 3 clinical study. No other COVID-19 vaccine candidates have yet advanced to late-stage testing. Swaminathan specifically noted "how advanced they are" and "the stage at which they are" in naming the vaccine being developed by AstraZeneca and Oxford as the likely leader.

Second, Swaminathan said that she thinks "AstraZeneca certainly has a more global scope at the moment in terms of where they are doing and planning their vaccine trials." The late-stage testing of AZD1222 will be conducted in several countries, with trials already in progress in the United Kingdom, Brazil, and South Africa.

The vaccine candidate was originally developed by the University of Oxford's Jenner Institute. AstraZeneca teamed up with Oxford in April and owns the rights to distribute the COVID-19 vaccine globally. AZD1222 was one of only a handful of novel coronavirus vaccines selected by the Trump administration to receive federal funds as part of Operation Warp Speed, an initiative to rapidly develop a safe and effective COVID-19 vaccine.

A close No. 2

Another COVID-19 vaccine is a close No. 2, according to the WHO chief scientist. Swaminathan views Moderna (MRNA -0.58%) as "not far behind" AstraZeneca and the University of Oxford in the race to develop a COVID-19 vaccine.

Moderna's messenger RNA (mRNA) COVID-19 vaccine mRNA-1273 is currently in phase 2 clinical testing. The biotech plans to begin a phase 3 study of the vaccine in July. Like AZD1222, Moderna's mRNA-1273 was included in the select group of COVID-19 vaccine candidates that are receiving federal funding in the Operation Warp Speed program.

Stephane Bancel, Moderna's CEO, is optimistic about the chances of success. He said in a CNBC interview recently that he believes that the probability of mRNA-1273 going on to win approval from the U.S. Food and Drug Administration is between 80% and 90%.

A crowded field

Investors should be wary of banking on AstraZeneca to emerge as the winner in the COVID-19 vaccine race just because it's the apparent leader right now. Nearly 150 COVID-19 vaccine candidates are currently being researched. Seventeen of those are in clinical trials, with more on the way.

It's impossible to know which, if any, of these experimental vaccines will be successful. It's not out of the question -- and perhaps even likely -- that multiple drugmakers will eventually win regulatory approvals for their COVID-19 vaccines.

The biggest winners from an investing perspective could very well be the small biotech stocks in the race. Novavax, for example, has a phase 1/2 clinical study underway for its COVID-19 vaccine candidate. BioNTech, like Moderna, is testing an mRNA COVID-19 vaccine with its partner, Pfizer.

AstraZeneca claims a market cap of nearly $70 billion. Success for its COVID-19 vaccine would definitely be a major catalyst for the stock. But success for a small drugmaker like Novavax or BioNTech would almost certainly be transformational for either company.