What happened

Shares of Moderna (MRNA 0.62%) and BioNTech (BNTX -1.30%) were slipping 4.3% and 3%, respectively, as of 3:51 p.m. EDT on Wednesday. The declines came after physicians in a presentation before an advisory panel to the Centers for Disease Control and Prevention (CDC) stated that the companies' COVID-19 vaccines were likely linked to cases of heart inflammation in adolescents and young adults.

So what

The CDC convened the Advisory Committee on Immunization Practices (ACIP) after reviewing initial data that showed higher-than-expected cases of heart inflammation after the second dose of messenger RNA (mRNA) vaccines. The numbers were especially notable in younger men.

Healthcare professional giving a shot to a young man.

Image source: Getty Images.

Moderna and BioNTech, with its big partner Pfizer (PFE 0.87%), have the only mRNA COVID-19 vaccines authorized for use in the U.S. Pfizer stock also fell slightly today, but the declines for the two other biotech stocks were greater mainly because Moderna and BioNTech don't have any products on the market other than their COVID-19 vaccines.

The reported cases of heart inflammation issues, while concerning, are rare. Tom Shimabukuro, deputy director of the CDC's Immunization Safety Office, stated in a presentation to ACIP that current data points to a rate of 12.6 cases per million in individuals between the ages of 12 and 39 within three weeks of receiving the second dose of the mRNA vaccines. 

Now what

The CDC hasn't indicated at this point what steps it might take in response to the potential issues with the Pfizer-BioNTech and Moderna vaccines. However, with the high efficacy and generally positive overall safety profiles for the vaccines, it's unlikely that any actions that would significantly impact the use of the vaccines will be taken.