A spokeswoman for the F.D.A., Stephanie Caccomo, declined to offer specifics. “While we cannot comment on individual interactions with sponsors, we do generally work with sponsors to ensure the number of participants in clinical trials are of adequate size to detect safety signals,” she said in an email message.
In June, the Centers for Disease Control and Prevention published data showing that the two vaccines may have caused myocarditis and pericarditis in more than 1,200 Americans, including about 500 who were younger than 30. The symptoms typically appeared within two weeks and were more common in young men and boys. The rate was low: Fewer than 13 cases per one million second doses administered.
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Most cases were mild and quickly cleared up, the researchers said. And Dr. Paul A. Offit, an infectious disease specialist who previously served on the C.D.C.’s Advisory Committee on Immunization Practices, which makes recommendations on vaccine use in the United States, noted that infection with the coronavirus also carries a risk.
If expanding the trials were to cause a delay in authorizing vaccines for pediatric use, he said, that would also put children at risk. “There’s always a human price to pay for knowledge,” he said. Of the heart ailments, he said, “It’s rare, it’s generally short lived and self resolving. It’s also a consequence of natural infection.”
The C.D.C.’s data showed 12.6 cases per million second doses administered, and researchers estimated that out of a million second doses given to boys ages 12 to 17, the vaccines might cause a maximum of 70 myocarditis cases, but would prevent 5,700 virus infections, 215 hospitalizations and two deaths. Covid-19 itself may cause heart problems in young people.
The F.D.A. authorized the Pfizer vaccine on an emergency basis for children ages 12 to 15 in April; so far, the Moderna vaccine has been cleared only for people 18 and older. The agency attached a warning about potential heart problems to the fact sheets of the vaccines in June.
Many public health experts argue that, with so much attention focused on hospitalizations and deaths among older Americans infected with the coronavirus, the risk for children has been overlooked. The American Academy of Pediatrics reported last week that more than four million American young children and adolescents have tested positive for the virus and 346 have died since the outset of the pandemic.
Mr. Biden’s attempt to put a general timeline on a possible authorization for children younger than 12 was unusual, and risked setting expectations for regulators to resolve a highly sensitive matter too soon. Mr. Biden and members of his administration have said they do not want to intervene in or be seen as influencing the work of regulators at the F.D.A. — something that former President Donald J. Trump tried to do repeatedly last year as the agency reviewed coronavirus drugs and vaccines.