Oxford University announced Friday it started testing its coronavirus vaccine in children as young as 6 in a move that expands coronavirus vaccine trials to the youngest age group yet.

The Oxford trial will include 300 child volunteers ages 6 to 17, with 240 of them receiving the vaccine co-developed with drugmaker AstraZeneca; the remaining participants will receive a control meningitis vaccine that has been proved safe in children but is expected to mimic side effects of a covid-19 shot, the university said in a statement.

Before the Oxford-AstraZeneca trial, testing had not included children younger than 12. Three other companies — Pfizer, Moderna and Janssen — have announced plans to start trials for younger children this spring.

Only the Pfizer-BioNTech and Moderna vaccines have been authorized in the United States thus far. Johnson & Johnson has a single-shot vaccine that could be authorized in March. U.S. regulators are awaiting more trial data before approving the Oxford-AstraZeneca vaccine, which is already in use in the European Union.

Even with clinical trials for younger patients underway, children are not expected to widely receive the vaccine for months and perhaps not until 2022. Richard Malley, a senior physician in the Division of Infectious Diseases at Boston Children’s Hospital, said he does not expect vaccinations in children to start until next calendar year

“We really want to make sure they’re safe and well-tolerated in children, particularly when there’s a low risk-benefit ratio,” Malley told The Washington Post on Saturday. The novel coronavirus has not shown itself to be as dangerous in most children as it is in older adults, making it less critical to hurry testing for children.

“The risk-benefit calculus you have to do should lead you to want to do this only if it’s extraordinarily safe in pediatric populations,” Malley said.

The Oxford trial’s small sample size of 300 children who span a decade in age is meant to serve as a jumping-off point and not the final word on how the vaccine will be tolerated by young patients.

Malley said that in a 300-person trial, the purpose is to determine what he called a “global response” to inform researchers at a high level whether the vaccine is safe, well-tolerated and able to produce an immune response in children.

Moderna, Pfizer and AstraZeneca have released promising results from their initial coronavirus vaccine trials. Here are answers to common questions about them. (Allie Caren/The Washington Post)

Although the development of various coronavirus vaccine candidates has occurred at an unprecedented speed, clinical trials expanding now to younger children follow the standard pattern of all vaccine testing, Malley said.

“In any vaccine study, you’re generally starting with healthy adults and slowly move into different age categories going up or down to make sure it’s safe for other adults,” Malley said.

For some health experts, the timeline of expanding clinical trials to teenage and younger patients has been disappointingly slow, particularly given the strength of the safety data for adults who have received the vaccine.

American Association of Pediatrics President Sally Goza wrote to federal leaders in September arguing that pediatric trials were essential for curbing the pandemic, given the potential of older children to be vectors for the virus.

“While some studies have shown that children under the age of 10 may be less likely to become infected and less likely to spread the virus to others, more recent data suggest children older than 10 years may spread SARS-CoV-2 as efficiently as adults,” Goza wrote.

Malley, the Boston Children’s Hospital doctor, said even though children are roughly half as likely as adults to transmit the novel coronavirus, inoculating younger populations is crucial to achieving herd immunity.

“When you read that kids are less likely to transmit, it’s roughly by a factor of two — so it’s not zero,” he said, noting that the virus variants that have emerged may further change the equation.

Malley also notes that although children broadly have not suffered severe or lingering illness from the novel coronavirus the way adults have, some children have developed acute respiratory failure and multisystem inflammatory syndrome, or MIS-C — a rare but serious illness that can be fatal or leave children with lasting heart damage.

So far in the United States, at least 11,000 children and teenagers have been hospitalized and at least 215 have died, according to a Jan. 28 report from the American Academy of Pediatrics.

“Even though these are rare, they can be catastrophic,” Malley said. “If the vaccine is safe and can be tolerated, it can save lives.”

Tara Bahrampour contributed to this report.

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