Pneumococcal Conjugate Vaccines (PCVs) are designed to prevent life-threatening diseases caused by the bacterium Streptococcus pneumoniae, including Pneumonia, Meningitis, Sepsis (Blood Infection), and Otitis Media (Ear Infection). Unlike older polysaccharide vaccines (like PPSV23), PCVs are "conjugated" to a carrier protein, which allows the immune system of infants and the elderly to recognize the bacteria more effectively and develop long-lasting "immune memory."