Moderna (NASDAQ:MRNA) is a white-hot COVID-19 stock just now. Onlookers have high hopes for mRNA-1273, the company's SARS-CoV-2 vaccine candidate, which is progressing rapidly through the development process. It's now well into phase 2 clinical testing, and the company has been given FDA approval to start a phase 3 study next month.

Many investors, it seems, are betting that the vaccine candidate will not only be successful, but successful early enough to garner a significant first-mover advantage. Given that nearly every rational human being on Earth will want and need vaccination against COVID-19, that market is set to be unimaginably massive.

It's an exciting possibility for Moderna, but investors have to bear a few things in mind once the dream wears off.

Businessman and businesswoman glancing at a screen to consider a decision.

Image source: Getty Images.

Firstly, as ever in the biotech world, we can't know if mRNA-1273 will ultimately prove efficacious. On top of that, Moderna is one of many companies and organizations working overtime to develop vaccines and treatments for COVID-19. Even mRNA-1273 works well enough to meet the requirements for a vaccine, other attempts could prove more effective, leaving Moderna at a disadvantage.  

Some of its rivals in this competition also have significant financial firepower, buttressed (like Moderna) by recent inflows of both public and private funds disbursed to aid the fight against the disease. A number also have global reach and a wide network of partners; I'm thinking in particular of Novavax and AstraZeneca, as well as pharmaceutical giants Pfizer and Merck.

So the decision whether to sell comes down to how much you, as an investor, rate Moderna's chances in this crowded race -- a race in which early leads can fade, and not every competitor will even cross the finish line.

If you feel mRNA-1273 has a good shot at not only sailing through its clinical trials and getting enough runway to be the COVID-19 vaccine for a good stretch of time, I'd say you should hold on to the stock. Otherwise, if you bought early, consider unloading at least some of your position and banking your considerable share price gains.