What happened

COVID-19 vaccine stocks came under enormous pressure last month. Over the course of August, Moderna's (MRNA -0.58%) shares dipped by a hefty 19.3%, Pfizer's (PFE 2.40%) stock ticked lower by a notable 10.4%, and Novavax's (NVAX 2.06%) equity lost an eye-catching 39.4% of its value, according to data from S&P Global Market Intelligence.

What caused investors to hit the exits on these top three coronavirus vaccine stocks? The central theme across these three healthcare stocks is the potential for a major drop in COVID-19 vaccine sales heading into 2023 and beyond.

So what

The U.S. government has already said that it will stop funding public programs covering the costs of coronavirus vaccines and therapies. What's more, U.S. health authorities recently stated that booster shots for healthy individuals will probably only be required once a year.

Previously, Wall Street believed that booster shots would be required every few months as new variants arose. As a result of these changing vaccine booster guidelines, this market might tumble in value over the next two years.   

That's not great news for Pfizer, Moderna, or Novavax from a revenue standpoint. Almost all of Pfizer's recent growth has stemmed from the COVID-19 vaccine Comirnaty and therapy Paxlovid.

Moderna and Novavax's near-term fortunes are also closely linked to their COVID-19 vaccines, Spikevax and Nuvaxovid, respectively. Novavax, in fact, cut its 2022 annual revenue guidance in half during its recent second-quarter earnings report -- a strong sign that demand for these vaccines is indeed starting to wane.  

On an important side note, Pfizer's stock was also stung last month by the ongoing litigation over the heartburn drug Zantac. That being said, the bigger threat to Pfizer's near-term outlook is clearly the future of its blockbuster COVID-19 product franchise.  

Now what

Are any of these beaten-down biotech stocks worth buying right now? Among the three, Pfizer comes across as the most compelling buy. Pfizer sports a rock-solid balance sheet, tons of deep value via its recent acquisition frenzy, an above-average dividend yield, and a well-rounded product portfolio outside of COVID-19. Pfizer's shares are also attractively priced at under 10 times forward-looking earnings right now.

In addition, Novavax's stock might also be a worthwhile value play at these levels. After all, the biotech's shares are currently trading at less than twice Wall Street's most conservative estimate for 2023 sales. In other words, Novavax's dramatic sell-off this year might be a tad overdone at this point.