Moderna's (NASDAQ:MRNA) stock has cooled off in recent weeks; currently at about $70, it's nowhere near the highs of more than $95 that it reached in mid-July. For investors who want to add the vaccine maker to their portfolios, now could be an appealing time to do so while it's cheap.

The public may know by the end of October whether Moderna's mRNA-1273 vaccine is effective against COVID-19. And you can be sure that if those results are positive, the stock will take off again.

The hype surrounding a vaccine is a big reason why Moderna's stock is up more than 250% this year, dwarfing the S&P 500 and its 4% returns thus far. But there's a lot riding on the company's current phase 3 trials of mRNA-1273, and a poor showing could send the stock crashing.

Let's take a look at whether today's a good time to invest in Moderna or if it's just not worth the risk.

Moderna's quietly continuing along with its trials -- a very good sign

In some cases, no news is good news. And that's the important takeaway for investors right now.

In September, AstraZeneca announced that it was putting its COVID-19 vaccine trial on hold after a participant in the U.K. experienced a spinal inflammatory disorder. And while the U.K. trials have since resumed, the one in the U.S. is on hold, as the Food and Drug Administration (FDA) here still has questions about the vaccine candidate's safety.

Another vaccine hopeful, Inovio Pharmaceuticals, is also going to delay the planned phase 2/3 trial of its vaccine candidate, INO-4800; the FDA has raised questions that Inovio needs to address before the trial can proceed. The stock crashed more than 20% on the news. Moderna, which launched its phase 3 vaccine trials on July 28, hasn't faced any such challenges thus far.

Woman sitting on bus using her smartphone and wearing a surgical mask.

Image source: Getty Images.

That doesn't mean everything's going perfectly for the Massachusetts-based business. But it's a good sign that two months in, there aren't any serious side effects derailing mRNA-1273's progress.

All this positions Moderna as a favorite to be one of the first companies to the finish line in the race for a COVID-19 vaccine. Its closest competition comes from Pfizer and BioNTech, which are collaborating on a vaccine candidate; they too could have results available from late-stage trials by as early as October.

But while finishing first is grand, it's not as important as getting an FDA-approved vaccine. Distribution of that vaccine will be another challenge, as there's significant demand all around the world -- too much for a single vaccine maker to fill right out of the gate. Whether Moderna is the first or second company with an approved vaccine may prove to be irrelevant, as there will still likely be strong sales to follow.

Is the stock worth the risk?

The one significant risk for investors today is that if Moderna releases unimpressive results from its study or is unable to get FDA approval for its vaccine candidate, the stock could crash in an instant. Inovio's stock crashed just on news that it would delay its trials. Imagine what a negative result from its trials could do to Moderna's share price.

With losses of at least $200 million in each of the past four years and sales of $60 million in 2019 down from $135 million in the previous year, Moderna doesn't give investors a whole lot of reasons to be bullish unless it gets a thumbs-up from the FDA for its COVID-19 vaccine candidate.

The company does have other vaccine candidates in its pipeline, although none are as far along as mRNA-1273. Moderna aims to start Phase 3 trials of its vaccine candidate for cytomegalovirus (a form of herpes), mRNA-1647, in 2021. If successful, it could generate as much as $5 billion in annual revenue at its peak. Its vaccine candidate for Zika virus, mRNA-1893, is also promising; while it's not yet in Phase 2, management says it presents a "several hundred-million-dollar annual peak sales opportunity."

Even with the stock's recent decline, it's still trading at an astronomical price-to-sales multiple of more than 200, which would be expensive by any benchmark:

MRNA PS Ratio Chart

MRNA PS Ratio data by YCharts

Investors are counting on that future sales growth, and without a successful vaccine, the company's not going to live up to that potential. Either way, when the results from the trial come out, the stock will either soar or crash, as there's not really much room for anything in between. The recent decline is not enough to make Moderna a safe investment or offset the risk investors would be taking if they were to invest in the stock.

Should you buy shares of Moderna today?

There's significant growth potential for Moderna, but that also depends on the success of the company's COVID-19 vaccine. If you're risk-averse, then you're better off looking at safer options than Moderna, as you could incur significant losses if the FDA doesn't approve its vaccine candidate.

But if you're OK with taking a chance, then Moderna could be a good buy right now. With the stock seemingly out of the spotlight recently and up just 3% over the past month (the S&P 500 is down 3% during the same time frame), it's an opportunity to invest just before what could be an incredible month for Moderna if it releases positive results from its trials. There's of course no guarantee that will happen, but if it does, it could produce significant returns for investors.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.