Pfizer is conducting a COVID-19 vaccine trial of about 44,000 participants 

It expanded its original trial size to include people of more diverse racial backgrounds and now intends to expand its age groups too 

As of Monday, Pfizer has 37,864 people enrolled, 42% of whom are over 65, and it will accept enrollment of children as young as 12 

Vaccines typically affect the immune systems of young and elderly people differently from the ways the affect immunity in younger adults adults 

Pfizer will enroll participants as young as 12 in its large, late-stage COVID-19 vaccine trial to understand how it works in a wider age group.

The US Food and Drug Administration (FDA) granted permission to the drugmaker and German partner BioNTech SE to enroll the younger participants this month, according to an update on Monday on the company's website.

The drugmaker is racing with rivals such as Johnson & Johnson, Moderna Inc and AstraZeneca to develop a safe and effective vaccine for the coronavirus.

The companies have pledged to ensure diversity in terms of race, ethnicity, gender, age and other factors in their vaccine studies.

Pfizer last month scaled up its trial to about 44,000 participants, from up to 30,000, to enroll people as young as 16 and those with chronic, stable HIV, hepatitis C and hepatitis B.