The Centers for Disease Control and Prevention (CDC) on Tuesday issued new COVID-19 mRNA vaccine guidance, expanding the recommended time between the initial two vaccine doses to eight weeks for some people over the age of 12, particularly young men.

The prior recommended interval between initial doses was three weeks for the Pfizer-BioNTech vaccine and four weeks for the Moderna vaccine. This timeline is still recommended by the CDC for individuals who are immunocompromised, over 65 years old or in need of rapid protection against the coronavirus.

According to the CDC, however, leaving more time between the first two doses of mRNA COVID-19 vaccines may reduce the risk for severe side effects such as myocarditis, or inflammation of the heart wall.

"MRNA COVID-19 vaccines are safe and effective at the FDA-approved or FDA-authorized intervals, but a longer interval may be considered for some populations," the CDC said. "While absolute risk remains small, the relative risk for myocarditis is higher for males ages 12-39 years, and this risk might be reduced by extending the interval between the first and second dose."

The recommended interval of at least five months between completing the initial vaccine series and receiving a booster shot remained unchanged in the updated guidance.

Earlier this month, the CDC had said in a presentation that it was considering extending the time between initial doses, citing data that showed the antibody response was higher when the time between shots was longer. Despite the potential side effects, the CDC maintained that the benefits of the mRNA COVID-19 vaccines "far outweigh risk of myocarditis."

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