Pneumococcal Disease

Types of Pneumococcal Disease

Pneumococcal disease is a significant public health concern worldwide. The bacteria responsible for this disease, Streptococcus pneumoniae, are commonly found in the respiratory tract of healthy individuals. However, under certain circumstances, they can invade other parts of the body and cause infections. Scientists have identified approximately 100 strains of Streptococcus pneumoniae. These strains are responsible for causing two main types of pneumococcal disease. The first type is noninvasive, which is more common, less serious, and does not spread to infect major organs or the bloodstream. The second type is invasive and is more severe. It can occur in the bloodstream, in areas of the body where bacteria are not normally found, such as the bones or brain, or in major organs like the lungs.

Fig. 1 Invasive and noninvasive pneumococcal disease syndromes. (O'Brien K L, et al., 2016)

Pneumococcal infections can manifest in different forms, depending on the area of the body affected. Common types of infection include:

• Ear Infections: Pneumococcal bacteria can cause infections in the middle ear, leading to painful symptoms such as earache, fever, and irritability, particularly in young children.
• Sinus Infections: When bacteria infect the sinuses, individuals may experience sinusitis, characterized by facial pain, nasal congestion, and thick nasal discharge.
• Pneumonia: Pneumococcal pneumonia is a severe respiratory infection that affects the lungs. It can cause symptoms such as coughing, chest pain, difficulty breathing, and fever. Pneumonia can be particularly dangerous for vulnerable populations, including the elderly and those with weakened immune systems.
• Bloodstream Infections: Invasive pneumococcal diseases can occur when bacteria enter the bloodstream, leading to severe complications. This can result in bacteremia (bacterial infection in the blood) or sepsis, a life-threatening condition.

Prevention of Pneumococcal Disease

Vaccines are the best way to prevent pneumococcal disease. There are two types of vaccines available to protect against pneumococcal bacteria:

• Pneumococcal polysaccharide vaccine
  The current pneumococcal polysaccharide vaccine is composed of purified polysaccharides from 23 serotypes. It primarily stimulates B-cells to produce IgM antibodies without the need for T-cell assistance. However, this immune response is not as robust as the response generated by conjugated vaccines. As a result, the polysaccharide vaccine is ineffective in children under 2 years old and often leads to non-response in older adults. Immunity provided by the vaccine is not lifelong, necessitating re-vaccination at age 65 for individuals initially vaccinated at age 60 or younger. Unlike conjugated vaccines, the polysaccharide vaccine does not induce mucosal immunity, impact carrier rates, promote herd immunity, or protect against upper or lower respiratory tract infections.
• Pneumococcal conjugate vaccines
  The pneumococcal conjugate vaccine is composed of capsular polysaccharides that are covalently linked to the diphtheria toxoid CRM197. This combination elicits a stronger immune response by recruiting type 2 helper T cells specific to CRM197. These T cells facilitate the production of non-IgM immunoglobulins and the generation of memory B cells. This immune response leads to the development of mucosal immunity and the establishment of lifelong immunity after multiple exposures. However, one limitation of conjugated vaccines is that they only protect against a subset of the serotypes covered by polysaccharide vaccines.

These vaccines offer protection against many strains of pneumococcal bacteria, but it's important to note that they do not cover all types. Additionally, while the vaccines provide good protection, they are not 100% effective. This means that even after vaccination, there is still a possibility of developing pneumococcal disease.