You might be kicking yourself that you didn't buy shares of coronavirus vaccine leader Moderna (NASDAQ:MRNA) earlier this year. The biotech's vaccine is now on the market in the U.S. and its market cap has skyrocketed.

But there are still other COVID-19 vaccines potentially on the way that could ultimately achieve commercial success. Their stocks could also be big winners, perhaps even bigger than Moderna's. It's not too late at all to get in on up-and-coming biotechs with promising vaccines in development. Actually, now could be the perfect time to buy one under-the-radar COVID-19 vaccine stock -- Altimmune (NASDAQ:ALT).

Hand holding blue marker with drawing of a clock that has "time to buy" written on it.

Image source: Getty Images.

Good news, bad news

If you're a pessimist, you might focus on the fact that Altimmune's shares have lost roughly two-thirds of their value since July 20, 2020. You could also worry that the U.S. Food and Drug Administration (FDA) placed a clinical hold on the company's application to begin a clinical study evaluating COVID-19 vaccine candidate AdCOVID.

It's true that Altimmune has had some bad news lately. However, the negatives don't outweigh the positives for the biotech.

Despite the decline in recent months, Altimmune's shares are still up close to 490% year to date. That's neck-and-neck with Moderna's performance, by the way. The slide in Altimmune stock through last week had more to do with overly exuberant investors driving the share price up too much too fast than with anything worrisome for the company.

What about that FDA clinical hold? Sure, it would've been better for Altimmune to have received a quick green light from the agency to advance AdCOVID into clinical testing. However, this clinical hold has nothing to do with concerns about the experimental vaccine. The FDA requested some tweaks to protocols planned for the clinical study and some additional manufacturing data. 

Altimmune stated on Dec. 23 that it has already responded to the FDA's clinical hold letter -- a quick turnaround, since the letter was received the previous day. The biotech agreed to the agency's requests. Altimmune doesn't "anticipate a significant impact on the overall clinical development timeline" at this point.

The news of the FDA's clinical hold caused the biotech stock to plunge 18%. However, I think the sell-off provides a buying opportunity for investors who believe in AdCOVID's potential. 

Potential remains huge

That potential is still huge. To understand why, just look at the limitations of the leading COVID-19 vaccines. 

Most of the coronavirus vaccines in development require two doses. This is the case for both of the vaccines that have already won FDA emergency use authorization (EUA). There are two obvious downsides to these two-dose vaccines. First, they're more expensive. Second, it's possible that some people won't go back to get their second dose and therefore won't receive maximum protection against COVID-19.

The top coronavirus vaccines also have cold chain requirements. For example, Pfizer's vaccine must be stored at ultracold temperatures. Other vaccines, including Moderna's, can be stored in standard refrigerators up to a point.

Also, the leading vaccines provide systemic immunity (meaning the body's primary immune system fights against infection). However, they could still allow the coronavirus to multiply in areas such as nasal mucus passages. 

Altimmune addresses all of these issues with AdCOVID. It's a single-dose intranasal vaccine that can be stored at room temperature for extended periods and will last in standard refrigerators for at least two years. As a bonus, AdCOVID appears to provide mucosal immunity against the coronavirus that causes COVID-19.

If AdCOVID proves both safe and highly effective, Altimmune just might emerge as a key player in the global coronavirus vaccine market. 

The big drawback

There's one big drawback for Altimmune, though: It's still way too soon to know if AdCOVID will deliver on its tremendous potential. We'll have to wait for the company to test its experimental vaccine in three clinical stages to learn whether that will be the case.

Because of this uncertainty, Altimmune won't be the kind of stock that's attractive to all investors. Its risk level will remain very high until more data is available for AdCOVID. For aggressive investors willing to take on these risks, though, now could be the best opportunity you'll have to buy this promising COVID vaccine stock.