It was unclear whether expanding the studies will have any impact on the timing of when vaccines could be authorized for children on an emergency basis.,

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At the F.D.A.’s urging, Pfizer-BioNTech and Moderna are expanding their studies of children 5 to 11.

Marisol Gerardo, 9, received a second dose of the Pfizer-BioNTech Covid-19 vaccine during a clinical trial for children at Duke Health in Durham, N.C., in April.Credit…Shawn Rocco/Duke Health, via Reuters

July 26, 2021, 12:30 p.m. ET

At the urging of federal regulators, two coronavirus vaccine makers are expanding the size of their studies in children ages five to 11 — a precautionary measure designed to detect rare side effects including heart inflammation problems that turned up in vaccinated people younger than 30.

Appearing at a televised town-hall-style meeting in Ohio last week, President Biden said that emergency clearance for pediatric vaccines would come “soon.” The White House has declined to be more specific on the timeline, and it was unclear whether expanding the studies will have any impact on when vaccines could be authorized for children.

Multiple people familiar with the trials said the Food and Drug Administration has indicated to Pfizer-BioNTech and Moderna that the size and scope of their pediatric studies, as initially envisioned, were inadequate to detect the rare side effects, including myocarditis, an inflammation of the heart muscle, and pericarditis, inflammation of the lining around the heart. Members of a C.D.C. advisory committee have said that the benefits of shots for people older than 12 greatly outweigh the risks, including of heart problems.

The F.D.A. has asked the companies to include 3,000 children in the 5-to-11 year old age group, the group for whom results were expected first, according to people familiar with the situation. One of the people, granted anonymity to speak freely, described that as double the original number of study participants envisioned.

A spokesman for Moderna, Ray Jordan, confirmed that the company intends to expand its trial “to enroll a larger safety database which increases the likelihood of detecting rarer events” and expects to seek emergency authorization in “winter 2021/early 2022.”

The Moderna trial began recruiting patients in March with the aim of enrolling 6,795 participants between the ages of six months and less than 12 years. Mr. Jordan said the company is “actively discussing” a proposal with the FDA.

Pfizer is on a faster timetable than Moderna, and may be able to meet the F.D.A.’s expectations on a bigger trial size and still file a request to expand emergency authorization of its vaccine by the end of September. Reviewing all the safety and efficacy data will likely take regulators at least a few weeks.

Pfizer has previously said it expects to have results for the 5-to-11-year old group in September, with results for children aged two to five shortly after. Results for the youngest children between the ages of six months and two years old are expected in October or November. A spokeswoman said Monday that the company had no updates on its timetable.

Questions about vaccinating children — including those under the age of 12 — are of huge interest to parents and teachers. Regulators will be required to balance potential side effects of coronavirus vaccination against the risks of Covid-19 itself.

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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College and universities. More than 400 colleges and universities are requiring students to be vaccinated for Covid-19. Almost all are in states that voted for President Biden.Hospitals and medical centers. Many hospitals and major health systems are requiring employees to get the Covid-19 vaccine, citing rising caseloads fueled by the Delta variant and stubbornly low vaccination rates in their communities, even within their work force. In N.Y.C., workers in city-run hospitals and health clinics will be required to get vaccinated or else get tested on a weekly basis.Can your employer require a vaccine? Companies can require workers entering the workplace to be vaccinated against the coronavirus, according to recent U.S. government guidance.

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.