Jennifer Macklom of Cedar Park, Texas, was determined to get her three daughters enrolled in a trial. Haunting her were the memories of 2016, when her daughter Miriam had grown sick with an adenovirus at age 2 ½. Simultaneously, her daughter Naomi, who was only 6 weeks old at the time, simultaneously contracted a form of the coronavirus that pre-dated Covid.

Miriam was taken by ambulance to the Dell Children’s Medical Center in Austin. Naomi was admitted through its emergency room.

Both girls were hooked up on oxygen and feeding tubes at the same time, Ms. Macklom recalled.

“I said, ‘I can’t do that again. I can’t emotionally have anyone I love or even know be hooked up to one of these things,’” said Ms. Macklom, a high school mathematics teacher.

So, with the advent of the pandemic, she conferred with her pediatrician, and all three children landed on a waiting list for a vaccine trial. Miriam is now 8, Naomi, 5, and Ruth, 2.