If you bought shares of Altimmune, (NASDAQ:ALT) Vaxart, (NASDAQ:VXRT) or Novavax (NASDAQ:NVAX) at the start of this year, you're already well on your way to riches. Thanks to the market's coronavirus vaccine frenzy, each of these companies' stock prices has exploded by more than 1,000% since the start of the year. But if you didn't invest early on, that's not surprising: Before the pandemic, these biotechs weren't exactly hot stocks.

In fact, none of them had a single drug or vaccine product approved for sale. Initiating COVID-19 vaccine programs was enough to propel them into the market's limelight, but each traveled a slightly different path to reach their current heights. Are any of them still good buys today?

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Altimmune

For the first two months of the year, Altimmune flew under the radar, its stock price rarely breaching $2. Even the savviest of investors couldn't have guessed that its stock would eventually grow by more than 1,160%. But at the end of February, the company announced that it had completed early development milestones for its coronavirus prophylactic, AdCOVID, and that it was initiating further preclinical testing, causing its price to nearly double overnight. During May, Altimmune stock grew from $2.96 to $9.28 on favorable AdCOVID news bundled into the quarterly earnings report, setting the stage for its next jumps.

After receiving a grant from the U.S. Department of Defense to develop its T-COVID therapeutic on June 29, Altimmune's stock swelled once again, expanding more than 200% in the two weeks that followed. Finally, the July 13 announcement of favorable results in its vaccine program buoyed the stock, taking it to the high-water mark of $33.26 on July 20. Since then, the stock has cooled off and is now trading in the $24 range, which may be a sufficient discount for investors seeking a bargain buy.

Vaxart

Vaxart's expansion story begins on Jan. 31, when it initiated its coronavirus vaccine program while its stock cost no more than $1.25 per share. A mere one month later, its stock had more than doubled in value, but even larger increases were to come. 

Despite a flurry of announcements about favorable preclinical data and the scaling up of manufacturing capacity in March and April, the stock wouldn't experience any similarly astonishing growth until the end of June. Being listed on the Russell 3000 index on June 24 helped Vaxart triple in value over the next several days, as did its announcement that the U.S. government's vaccine accelerator program had agreed to fund the company's efforts.

Shortly afterward, speculators fueled the rest of Vaxart's climb to $16.97 in mid-July, accounting for an increase of 4,748% over the start of the year. Though a large portion of these gains were sharply culled over the following weeks, the company's trajectory is still quite marvelous. At the beginning of 2020, Vaxart's stock was trading at $0.36. It's now up by about 2,490% and costs about $9 per share.

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Novavax

Novavax stock has posted the steepest growth of the three, up more than 3,240% from its price in the $4 range at the start of the year to reach its current level of around $131. This means that if you had invested $10,000 in Novavax in January, you'd have more than $300,000 right now. 

By late February, Novavax was knee-deep in vaccine development, and its stock was up more than 300%. That price increased steadily through April, and on May 11, the company announced that it had accepted $388 million in funding from the Coalition for Epidemic Preparedness Innovations, with clinical trials to start later that month. This prompted a rapid move beyond $56 per share. 

In June, Novavax stock really started taking off, but the big news came July 7 when investors learned that the U.S. government had invested in its coronavirus vaccine program to the tune of $1.6 billion. Positive clinical trial results for its vaccine candidate brought the stock to a high of more than $178 in early August, and while it's declined some since then, it just might have a few more growth spurts in store before the end of the year. Keep an eye on Novavax's clinical trial results to see whether it can break its prior record.

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.