Pfizer will start testing its coronavirus vaccine on children as young as 12, a crucial step to bringing the shot to more people

• Pfizer is expanding its massive coronavirus vaccine clinical trial, opening the study to include children as young as 12.
• The company is poised to become the first major drugmaker to start testing a COVID-19 shot in kids in a large-scale study.
• The New York pharma giant said it got permission to boost the study's overall size to as many as 48,400 volunteers.
• Pfizer broadened the study last month to include 16- and 17-year-olds as well as people with HIV, hepatitis B, or hepatitis C.
• While early clinical results were promising, it remains to be seen if Pfizer's vaccine — or any other experimental shot — can prevent COVID-19.
• Pfizer CEO Albert Bourla has said he expects data from this study before the end of October.

Pfizer is setting plans to make its coronavirus vaccine available to more groups of people, should the pharma company prove that the shot works to prevent the virus.

On Monday, the company said on its website that it got approval from regulators to expand a trial of the shot to include children as young as 12, becoming the first major drugmaker to open a coronavirus vaccine trial to kids. A Pfizer spokesperson said it also got permission to expand the study to include as many as 48,400 volunteers, 10% larger than the previous cap.

The New York-based firm expanded the trial in September to include 16- and 17-year-olds, as well as people with hepatitis C, hepatitis B, or HIV.

Pfizer is a frontrunner in developing a coronavirus vaccine. It started the final stage of trials in July and has said it expects to know whether the shot works by the end of this month.

So far, vaccine research has predominantly focused on adults. That means kids probably won't be able to get a coronavirus vaccine at first.

Read more: Drugmakers still haven't started testing their coronavirus vaccines in children, putting kids at the end of the line for a potential shot

There are two reasons the vaccine research started with adults.

First, vaccines are usually tested gradually, beginning with healthier and less-vulnerable people. More vulnerable populations are added to trials as data mounts that shows the shots are safe and that they work.

Secondly, COVID-19 has shown to hold the most risk for the elderly, with younger people faring much better in recovering from the virus.

But having a vaccine for kids will still be vital in fighting this pandemic, particularly in helping schools safely reopen.

Moderna, a Massachusetts-based biotech, has said that it plans to start a pediatric trial for its shot before year's end, pending approval from regulators. Moderna started the final stage of testing in July and expects to have efficacy results in November.

Read more: Coronavirus-vaccine frontrunner Moderna is planning to start testing its shot in kids — a crucial step to halting the pandemic

Johnson & Johnson Chief Scientific Officer Paul Stoffels said J&J is committed to running pediatric trials. Opening studies for children "will come later in the year" after safety is established in adults, Stoffels said on a September 22 call with reporters. J&J started the final stage of trials in September.

Moncef Slaoui, the chief scientist overseeing the US government's coronavirus vaccine initiative, told Business Insider he believes a vaccine needs to show it can work in adults before being tried in large numbers of children.

"From a benefit-risk standpoint, I do think that we should have evidence that vaccination can be successful before embarking into large trials in pediatric," Slaoui said.

Slaoui expects leading coronavirus vaccines would be tested in populations that gradually go down in age, from adolescents to toddlers and eventually to babies, contingent on positive results.

Slaoui estimated that if pediatric studies started in January, they could "come to conclusion somewhere in April or May."