Pneumococcal Vaccines

When a person's immune system first comes in contact with the pneumoccocal bacteria, the immune system does not immediately recognize the threat. Because of this, the immune response is slow, giving the bacteria a head start of several days. In the few days it takes for the immune system to mount a response, life-threatening illness can occur, especially in young children and the elderly. Luckily, the immune system has a complex and strong memory. If a person is exposed to the same strain of pneumococcus a second time, the immune system immediately recognizes the threat and reacts swiftly to eliminate the bacteria often before it can even cause illness. This immune memory is why most people can only get many diseases once, for example chicken pox. When a person is vaccinated against pneumococcal disease, they are injected with small parts of killed bacteria that can be recognized by the immune system, but that are completely incapable of causing disease. The immune system learns to recognize the pneumococcus without being exposed to its dangers. Later, if the vaccinated individual encounters the bacteria in society, he or she will mount an immediate response and eliminate the bacteria without getting sick.

The pneumococcal polysaccharide vaccine (PPV) exposes the immune system to polysaccharide chains (sugars) that help makeup the bacterial capsule. The PPV vaccine is effective against 23 serotypes of pneumococcus that cause 88% of invasive pneumococcal disease (Immunization Action Committee). Because polysaccharides are weakly immunogenic, the PPV vaccine can work well in the elderly, but does not work as well in young children whose immune systems are very naïve.

The pneumococcal conjugate vaccine (PCV) exposes the immune system to the same polysaccharides as the PPV vaccine but first the sugars are conjugated (attached) to a more easily recognized part of another organism. This vaccine is capable of inducing memory in the immune system and is the vaccine that is used for children.

There is a third type of vaccine that is currently being researched, protein vaccines. Protein vaccines induce immunity by exposing the immune system to inactive proteins that are derived from the pneumococcus itself. These vaccines have shown promise in animal models and, if they can be made safe and effective, would be an excellent addition to the vaccine repertoire because they would be effective against all serotypes of the disease.