A "potential adverse event" related to a coronavirus vaccine has been detected among a small portion of predominantly male teenagers and young adults.
The young vaccine recipients, based in the United States, experienced inflammation in their heart after getting one of two mRNA jabs - Moderna or Pfizer - which were recently given emergency authorisation.
The Centres for Disease Control and Prevention Advisory Committee on Immunisation Practices revealed on May 17 it had investigated reports a portion of young recipients developed myocarditis.
The individuals were predominantly adolescents and young adults, and predominantly male, according to a statement.
Myocarditis, an inflammation of the heart muscle, normally goes away on its own and can be caused by several viruses.
The cases detected were not beyond what could be expected in the population, the CDC said, but it felt healthcare providers should be informed of reports of a "potential adverse event".
It was not revealed how many people were affected by the heart condition or which vaccine they received, with the CDC recommending the matter be investigated further.
The cases typically developed the heart condition within four days of receiving the vaccine, the CDC said.
The US authorised the Pfizer vaccine for children aged 12 to 15 earlier this month.
Australia's increasing access to Pfizer doses is expected to allow for everyone who wants the vaccine to receive it by the end of the year.
Health Minister Greg Hunt this week faced backlash after suggesting that people over 50 hold off receiving the AstraZeneca vaccine, with supply of alternatives scheduled to increase later in the year.
"Right now, we want to encourage everybody over 50 to be vaccinated as early as possible," he said, before offering a caveat.
"But we've been very clear that, as supply increases later on in the year, there will be enough vaccine of mRNA vaccines [Pfizer and Moderna] for every Australian."
His comments drew heavy criticism given the country's vaccine rollout is already months behind schedule.
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