Stanford Health Care has begun laying the clinical groundwork to expand COVID-19 vaccine eligibility to the country's youngest children, joining in a nationwide trial to see how safe and effective the Pfizer vaccine is for people ages 2 to 5.

The early-phase study, which involves 144 children, is taking a close look at how young children respond to the Pfizer vaccine and whether they can handle the same dose size or experience the same side effects as adults. No vaccines are currently approved for those under 16 years old, and public health officials say extending the vaccine to children will be a key factor in curbing the spread of the virus.

Pfizer is already moving full steam ahead to get the vaccine approved for use in children ages 12 to 15, and the Food and Drug Administration is expected to approve the vaccine's use in children in that age category either this month or in May, said Dr. Yvonne Maldonado, professor of pediatric infectious diseases and of health research and policy at Stanford.

But still in its infancy are trials to determine whether the vaccine remains both safe and effective in children under age 12, which is where Maldonado said Stanford is currently focused. Participants are getting the same Pfizer regimen — two doses three weeks apart — but the first cohort of children are getting a smaller dose. Researchers will test blood samples to check for antibodies and closely monitor patients for signs of a fever, headache, muscle aches and other possible side effects before ramping up dose sizes for later cohorts.

It's too early to say what the results will be, but Maldonado said she's optimistic.

"We know that Pfizer did submit data for 12- to 15-year-old (children), and apparently at the adult dose, they did get a very robust immune response," she said. "We're hoping that will be true for the younger kids as well."

Children will be the last group of people eligible for the vaccine, partly due to the nature of COVID-19 and the threat it poses to them. Early on in the pandemic and at the start of vaccine development, the belief was that children didn't get infected, and if they did, they didn't get sick, Maldonado said.

Those views, coupled with the decision to shut down schools and cancel social events, created a perception that children were immune to the virus. But the thinking has changed since then, Maldonado said. Children do get sick, and they are at risk of serious long-term symptoms after contracting the virus.

"There have been thousands of hospitalizations, and over 300 children have died," she said. "It is deadly and fatal in children. It's not common, but it's not rare either."

Data from the Centers for Disease Control and Prevention (CDC) show that more than 2.4 million children age 5 to 17 have tested positive for COVID-19 in the U.S., followed by nearly 500,000 children ages 0 to 4. That's a total of 2.9 million cases, or 11.9% of all cases in the country.

Researchers are moving fast to extend vaccine eligibility to children in an effort to reach so-called herd immunity, in which at least 70% of the population has been immunized and largely prevents the virus from spreading. Census data shows children under age 18 make up close to a quarter of the population, making them an essential part in reaching that target.

Stanford is expected to participate in trials involving older children later this spring, and the current forecast is that the Pfizer vaccine will be available for children under age 12 by 2022.

"We have pretty strong evidence that approval for (those) 12 to 15 years old might happen before the summer, we're hoping. But for those under 12, it's not likely we'll see something before the end of the year," Maldonado said.

Though the first phase of the Pfizer trial at Stanford is already fully enrolled with volunteers, residents can still sign up to participate in future phases. Anyone interested can apply online redcap.stanford.edu.

Find comprehensive coverage on the Midpeninsula's response to the new coronavirus by Palo Alto Online, the Mountain View Voice and the Almanac here.