What happened

Shares of Moderna (MRNA -1.11%) fell 9% on Wednesday after Scandinavian health officials said they would take action to limit the use of the drugmaker's COVID-19 vaccine in children and young adults. 

So what

The Swedish health agency will hold off giving Moderna's drug to people under 30 years old after preliminary data from a not-yet-published study showed a potential higher risk of heart inflammation in those who received the vaccine. "The connection is especially clear when it comes to Moderna's vaccine Spikevax, especially after the second dose," the agency said. Swedish health officials did, however, note that the condition was very rare.

Denmark will likewise suspend the administration of Moderna's vaccine in kids aged 12 to 17.

A medical professional is administering a vaccine to a child.

Moderna could lose vaccine sales to its rivals. Image source: Getty Images.

In its place, the two countries will recommend Pfizer (PFE 0.91%) and BioNTech's (BNTX 0.29%) COVID-19 vaccine. Additionally, Norway reiterated its recommendation that children and young men use Pfizer and BioNTech's drug due to side effects from Moderna that appear to be more prevalent in young males:

"Men under the age of 30 should also consider choosing Cominarty when they get vaccinated," Norwegian health official Geir Bukholm said.

Now what 

It's not good news when medical professionals recommend a rival's drug in place of your own. But Moderna's investors should note that more data from the aforementioned Nordic study is expected in the coming month. The European Medicines Agency (EMA) will review the data to see if additional -- or, perhaps, fewer -- precautions are warranted.