S. pneumoniae?bacteria are serotyped based on the polysaccharides in the outer capsule of the bacteria. The more than 100 known serotypes vary in how common they are and in what percentage of pneumococcal disease they cause. Pneumococcal vaccines are designed to target specific serotypes.

Our understanding of which serotypes are currently causing invasive pneumococcal disease (IPD) in the United States comes primarily from CDC’s Active Bacterial Core surveillance (ABCs) program, which routinely collects serotype data on IPDs leading to hospitalization in 10 states. From ABCs and similar systems in Alaska and the Navajo Nation, CDC has determined that certain adult populations in the western United States have a high percentage (30% or higher) of IPD caused by serotype 4. These areas include Alaska, Colorado, the Navajo Nation, New Mexico, and Oregon. Serotype 4 is not commonly detected in other regions of the United States.

Typically, people in these geographic areas who develop serotype 4 IPD are adults younger than age 65 years who have specific underlying conditions or risk factors, such as alcoholism, chronic lung disease, cigarette smoking, homelessness, and injection drug use.?Affected adults typically have not received a pneumococcal vaccination targeting serotype 4.

Serotype 4 is included in all but one of the current pneumococcal vaccines offered to children and adults (PCV13, PCV15, PCV20, and PPSV23). Serotype 4 is not included in the adult PCV21 (Capvaxive, Merck). While ACIP has not expressed a preference for a specific pneumococcal vaccine schedule for adults, ACIP notes that vaccine schedules that include serotype 4 (PCV20 alone or PCV15 followed by PPSV23) are expected to provide broader serotype coverage for adults in these western states with underlying conditions or risk factors for serotype 4 IPD. ACIP and CDC have indicated they will continue to monitor the prevalence of serotype 4 and provide additional guidance if necessary.

For more details of clinical considerations for selection of pneumococcal vaccines in communities with high proportions of serotype 4, please see the box on page 797 of the September 12, 2024, MMWR article entitled, Use of 21-Valent Pneumococcal Conjugate Vaccine Among U.S. Adults: Recommendations of the Advisory Committee on Immunization Practices — United States, 2024: www.cdc.gov/mmwr/volumes/73/wr/pdfs/mm7336a3-H.pdf.