This study is designed to check if a new medicine called crovalimab is safe and works well for children with a rare disease called atypical hemolytic uremic syndrome (aHUS). aHUS is a condition where small blood clots form in the blood vessels, leading to problems like kidney failure.

• Participants will need to be vaccinated against certain infections before joining.
• Some participants may continue other treatments if they have been on the same dose for at least 28 days.
• People with certain infections or past medical conditions may not be eligible to join.

Children must weigh at least 5 kg (around 11 pounds) to join. Some participants may have had a kidney transplant or have taken specific treatments before. To ensure safety, participants will be closely monitored. The study will also check how crovalimab moves in the body (called pharmacokinetics) and how it affects the body (called pharmacodynamics). Participants should not be on dialysis recently or have other specific health issues to join. If you're interested, discuss with your healthcare provider to understand the risks and benefits.