In a potentially alarming development, the U.S. Centers for Disease Control and Prevention (CDC) said Thursday that young American men are showing notable increases in heart inflammation conditions after receiving their final coronavirus vaccine shot.

That's according to data from a pair of vaccine monitoring systems, which indicate that over 50% of post-inoculation heart inflammation cases occurred in males aged 12 to 24. Meanwhile, people in that demographic received only 9% of the two-shot regimens of the Moderna (MRNA 1.58%) or Pfizer (PFE 0.66%)/BioNTech (BNTX -2.07%) vaccines, so the imbalance is clear.

Young man about to receive a vaccine shot.

Image source: Getty Images.

By far, those heart inflammation cases occurred within one week of the recipients getting their second shot. Other data show that the median age of vaccinated individuals who developed heart inflammation following their final dose was 24.

The CDC is convening a meeting of its advisory committee on immunization practices next week to review the evidence and evaluate the risk of such conditions.

In a media statement, Pfizer wrote, "It is important to understand that a careful assessment of the reports is ongoing and it has not been concluded that the mRNA COVID-19 vaccines cause myocarditis or pericarditis." 

The company has also pointed out that "the number of reports is small given the number of doses administered." All told, around 130 million Americans have been fully inoculated with either the Moderna or Pfizer/BioNTech vaccine.

In an article on the development published Thursday by Reuters, the news agency wrote that Moderna said it hasn't established a causal association between heart inflammation and the administration of mRNA-1273, its coronavirus vaccine.

Investors in the three affected companies had mixed reactions to the news. Shares of Moderna and BioNTech slumped on the day, against the S&P 500's 0.5% growth, while Pfizer rose by 2.2%.