This study is testing if a new vaccine called **PCV21** is safe and helps children and teens with sickle cell disease (a blood disorder) make antibodies. Antibodies are the body's defense against germs. The study will compare PCV21 with another vaccine called **20vPCV**, which is already approved for use.

Eligibility: Children and teens aged 2 to 17, with sickle cell disease, who may or may not have had previous vaccines for pneumonia, can join. Participants must not have certain medical conditions or have taken certain medications recently. They must also not be pregnant or breastfeeding.

Key Points:

• The study lasts up to 6 months.
• Participants must attend all scheduled visits and follow study guidelines.
• Certain health conditions or prior treatments may affect eligibility.

The study will check for any side effects and if the vaccine works well. Participation requires parental consent and involves regular health check-ups. If you have sickle cell disease and meet the criteria, this study might be an option for you.