This study is testing a new shot called the PCV21 vaccine to see if it's safe and helps kids' bodies fight germs. The vaccine is for kids and teens aged 2 to 17 years with sickle cell disease, a condition where red blood cells are not shaped right, causing problems in blood flow. The study lasts up to 6 months.

Participants will get just one dose of the vaccine. It aims to see if this vaccine creates "antibodies", which are like tiny warriors in the body that fight infections. The new vaccine is compared to another vaccine called 20vPCV, which is already approved.

To join, kids must be between 2 and 17 years old, have sickle cell disease, and not be pregnant. They must agree to attend all visits and follow study rules. They cannot join if they have certain medical problems, have had certain medicines, or have taken other vaccines recently.

• Study duration is up to 6 months.
• Participants need one vaccine shot and a few visits.
• Check if any health issues might prevent participation.