What happened

Shares of Moderna (MRNA -2.45%) fell on Friday, following a major development in the war against COVID-19. By the close of trading, the biotech's stock price was down 16.6% after falling as much as 25.7% earlier in the day.

So what

Rival drugmaker Pfizer (PFE -3.85%) announced that its clinical study results indicate that its investigational antiviral treatment could slash the risk of hospitalization or death by 89% in non-hospitalized adults with COVID-19. 

"Today's news is a real game-changer in the global efforts to halt the devastation of this pandemic," Pfizer CEO Albert Bourla said in a press release. "These data suggest that our oral antiviral candidate, if approved or authorized by regulatory authorities, has the potential to save patients' lives, reduce the severity of COVID-19 infections, and eliminate up to nine out of 10 hospitalizations."

Pfizer said it intends to submit the data to the U.S. Food and Drug Administration and request Emergency Use Authorization for the drug "as soon as possible."

A downwardly sloping stock chart.

Image source: Getty Images.

Now what

Investors appear to be concerned that more people may choose not to get vaccinated if Pfizer's pill is proven to be as safe and effective as its clinical trial results suggest. In that scenario, Moderna's vaccine sales could fall dramatically in the coming years.

However, health officials were quick to caution that Pfizer's treatment is not designed to replace vaccinations. Vaccines can help prevent disease, while Pfizer's pill can only lessen the negative effects of getting sick.

Thus, Moderna's projections of $17 billion to $22 billion in sales in 2022 may still be achievable -- and today's sell-off could turn out to be an overreaction on the part of investors.