Kaiser Permanente officials announced Tuesday that they are expanding COVID-19 vaccine trials to include children ages 5-11 in Oakland, Santa Clara and Sacramento.

The three trial sites on Monday began enrolling roughly 75 children in the 5-11 age range.

Kaiser's Vaccine Study Center and Division of Research, both based in Oakland, are conducting the trial in concert with Pfizer and BioNTech, which co-developed the vaccine that will be used in the trial. The Pfizer vaccine is currently the only COVID-19 vaccine approved for minors.

Roughly 4,600 children are participating in trials across the country to determine the vaccine's safety and efficacy in children under 12.

"There's a lot of vigorous interest right now in terms of availability and there's limited availability" for the study, said Vaccine Study Center director Dr. Nicky Klein.

Kaiser's Vaccine Study Center also participated in the clinical trials to determine the safety and efficacy of the Pfizer vaccine both in adults and children ages 12-17.

In the coming weeks, trials are also likely to open for children under age 5, Klein said, as logistics and finer details are still being worked out. Eventually, she said, medical experts intend to recommend that all children receive the vaccine once it is available to them.

On the trial's current timeline, Klein said Pfizer could submit an emergency authorization for children 5-11 as early as September, provided the trial's data shows the vaccine is as effective as it has been for other age groups.

"It may be around the start of school," she said. "(That's) probably the best-case scenario, but I think that's still up in the air."