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 had tested positive for the virus and 349 had 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.
Government regulators and scientists have been hamstrung in recent months by a patchwork of sometimes weak data collection systems in the United States for vaccine side effects, forcing them to lean on a program called the Vaccine Adverse Event Reporting System, a 30-year-old federal monitoring system that relies on self-reported cases from patients and health care providers. That dilemma has increased the pressure on companies to do more rigorous studies in children.
“There is a clear and well-defined risk to delaying the availability of these vaccines” in young children, said Dr. Steven Black, an emeritus professor of pediatrics at Cincinnati Children’s Hospital Medical Center and the co-director of the Global Vaccine Data Network, a consortium that researches the safety of vaccines.
Dr. Black said that because of how rare the conditions were, increasing the size of the studies might not allow regulators to better detect the prevalence of the heart problems.
“The only way we’ll know about that is to roll the vaccine out and see what happens,” he said. “Passive reporting will probably be very good because I think we’re sensitized to the issue. We know to look for it. Physicians know to look for it.”