The purpose of this study is to measure whether PCV21 vaccine (investigational pneumococcal vaccine) is safe and can help the body to develop germ-fighting agents called "antibodies" (immunogenicity) compared with 20vPCV (licensed pneumococcal vaccine) when given as a single dose to children aged 2 to 17 years with sickle cell disease who had received or not a previous vaccination with pneumococcal conjugate or pneumococcal polysaccharide vaccine.