Oxford-AstraZeneca begins a vaccine trial for children. It's the youngest group yet to be tested.

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 proven safe in children but is expected to mimic similar 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 still waiting for 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 may 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 Saturday. Coronavirus has not shown itself to be as dangerous in most children as it is in older adults, making it less critical to race out 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," he 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 in a 300-person trial, the purpose is to determine what he called a "global response" and tell researchers at a high level if the vaccine is safe, well-tolerated and able to produce an immune response in children.

While the development and rollout 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 taken the vaccine.

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

"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 an adult to transmit 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 points to the fact that while children broadly have not suffered severe or lingering illness from covid-19 the way adults have, some children have developed acute respiratory failure and multisystem inflammatory syndrome, or MIS-C - a rare but serious illness than 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 215have 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."