To reduce the rare risk of heart disease, the U.S. Centers for Disease Control and Prevention recommended this week that young men wait longer between Pfizer and Moderna vaccinations.

The CDC recommended that males aged 12-39 years should wait eight weeks between their first and second doses. Canada’s public health experts found that men aged 18-24 had a lower risk of developing myocarditis if they wait eight weeks before receiving the second doses of Moderna and Pfizer.

The CDC suggests that eligible patients wait for three weeks after receiving Pfizer shots, and four weeks before taking Moderna doses. This is especially important for the elderly or those who have compromised immune systems.

The National Heart, Lung and Blood Institute explains that myocarditis, which is inflammation of heart muscle, can cause serious health problems. Myocarditis occurs most often after viral infections. However, myocarditis can also be caused by Moderna or Pfizer shot, especially the second.

Myocarditis risk for men aged 18-39 is 1.5 times greater after receiving a second Moderna vaccine than with Pfizer. This age group has an average of 68 cases of myocarditis per 1,000,000 Moderna doses, as opposed to 47 cases for 1 million Pfizer doses.

The CDC reports that most people who get myocarditis from Covid vaccines respond to treatment and are fully healed. The Department of Health and Human Services says that myocarditis rates are much higher after Covid than they were from vaccines.