What happened

This isn't shaping up to be a good week for the stocks of leading COVID-19 vaccine makers. Several of the stocks slipped on Monday. The downtrend continued today.

Shares of Pfizer (PFE 0.91%) were down 1.6% at 11:27 a.m. BioNTech (BNTX 0.29%) stock had fallen 9.4%. Moderna's (MRNA -1.11%) shares were 6.2% lower. Novavax (NVAX 1.16%) was an especially big loser, its shares having sunk 11.6%.

The declines for these vaccine stocks came despite some good news for several of them. Pfizer and BioNTech submitted initial data to the U.S. Food and Drug Administration from a late-stage study of their COVID-19 vaccine in children younger than 12 years down to age 5. Indian regulators also allowed the Serum Institute of India to begin enrolling kids ages 7 to 11 in a clinical trial evaluating Novavax's COVID-19 vaccine.

So why are the stocks falling today? It's probably because investors are becoming increasingly anxious about the prospects for COVID-19 vaccine sales. The CEOs of both Pfizer and Moderna recently predicted that the pandemic could end in 2022.

Also, Pfizer announced on Monday that it's started testing a pill that could be used to prevent infection after exposure to another person with COVID-19. If Pfizer's oral drug or similar antiviral therapies become available, it could reduce the incentive for people to seek vaccination.

A healthcare professional holding a syringe and needle up to a vaccine vial.

Image source: Getty Images.

So what

Investors absolutely should be looking to the future to evaluate the prospects for vaccine stocks. And there is uncertainty as to whether COVID-19 vaccine sales will remain as strong as they are now in 2023 and beyond.

It's not surprising that Pfizer stock didn't fall nearly as much as shares of BioNTech, Moderna, or Novavax. The big drugmaker isn't totally dependent on its COVID-19 vaccine, as BioNTech and Moderna are -- and Novavax potentially soon will be.

However, there's still a possibility that emerging coronavirus variants could cause the demand for COVID-19 vaccines to remain strong for years to come. Today's sell-off could turn out to be unwarranted.

Now what

Each of these stocks has key catalysts on the way. Pfizer and BioNTech hope to soon file for FDA Emergency Use Authorization for their COVID-19 vaccine in immunizing younger children. Moderna awaits a decision on booster doses. Novavax hopes to win Emergency Use Listing from the World Health Organization for its COVID-19 vaccine and plans to file for EUAs in multiple countries.

As long as the uncertainty about future demand for COVID-19 vaccines continues, though, look for continued volatility with all of these stocks.