What happened

Moderna's (MRNA 0.38%) stock dipped by more than 6.7% as of 2:45 p.m. ET on Thursday, with the biotech announcing that regulators in Canada had approved its Spikevax coronavirus vaccine for use in toddlers between the ages of 6 months to 5 years.

While the approval is favorable news, it was likely overcast by even more favorable news from a soon-to-be major competitor, Novavax (NVAX -1.87%), which on the evening of July 13 got an Emergency Use Authorization (EUA) for its jab in the U.S. for those 18 and older.

So what

With Novavax's candidate in play, Moderna will now face a harder fight for vaccine market share in the U.S., and that could mean making fewer sales over the next few years or being forced to increase research and development expenditures to stay relevant. Management's estimate is that Spikevax revenue for 2022 will be at least $21 billion, which is derived from selling booster shots, as well as doses for initial vaccination. At the moment, Novavax's entry is only approved for the primary series vaccination rather than for use as a booster dose, so Moderna will continue to split the U.S. booster market with Pfizer for the time being. 

Now what

Successfully competing against Novavax may be a matter of beating it to the market with variant-specific booster shots, one of which, called mRNA-1273.214, is already in late-stage clinical trials. But if the updated jab isn't significantly more effective than the competitor's non-updated candidate that was just approved, it'll be a strong headwind that'll depress sales, potentially sharply. 

Moderna will report its Q2 earnings on Aug. 3 before the market opens. Investors should be on the lookout for any discussion of how the variant-specific vaccine is faring in clinical trials, as it'll be a key consideration for government purchasers down the line.