Pneumococcal disease is caused by Streptococcus pneumoniae, a bacterium that has more than 100 serotypes. Most serotypes can cause disease, but only a few produce the majority of cases of invasive pneumococcal disease.
The pneumococci bacteria are spread from person to person by droplets in the air. The pneumococci bacteria are common inhabitants of the human respiratory tract. They may be isolated from the nasopharynx of 5%–90% of healthy people.
Pneumococcal disease can be invasive, meaning a normally sterile part of the body is infected, or non-invasive. There are two major clinical syndromes of invasive pneumococcal disease (IPD): bacteremia (blood stream infection), and meningitis (infection of the meninges that surround the brain). They are both caused by infection with the same bacteria but produce different signs and symptoms.
Pneumococcal pneumonia is a common disease caused by pneumococcal infection. Symptoms include abrupt onset of fever, shaking chills or rigors, chest pain, cough, shortness of breath, rapid breathing and heart rate, and weakness. The case-fatality rate is 5%–7% and is higher in adults 65 years and older and people with certain underlying medical conditions.
Pneumococcal pneumonia can occur in combination with bacteremia and/or meningitis (invasive pneumococcal pneumonia), or it can occur alone (non-invasive pneumococcal pneumonia). Before the COVID-19 pandemic, at least 100,000 people were hospitalized each year for pneumococcal pneumonia. At least 30,000 people were hospitalized each year for IPD, and about 3,000 people died. Rates of pneumococcal disease declined during the COVID-19 pandemic, but rates have returned to pre-pandemic levels.
About 4,000 cases of pneumococcal bacteremia without pneumonia occur each year in the United States. Bacteremia is the most common clinical presentation among children less than two years, accounting for up to 70% of IPD in this age group. The overall case-fatality rate of pneumococcal bacteremia is about 20% and may be as high as 60% among older adults.
Pneumococci cause 50% of all cases of bacterial meningitis in the United States. There are an estimated 2,000 cases of pneumococcal meningitis each year. Symptoms and signs may include headache, tiredness, vomiting, irritability, fever, seizures, and coma. The case-fatality rate of pneumococcal meningitis is about 8% among children and 22% among adults. Permanent neurological damage is common among survivors.
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.
Information about pneumococcal serotype prevalence is not available for most communities. The best available data is from communities in the 10 states that have active pneumococcal surveillance programs through the CDC’s Active Bacterial Core surveillance (ABCs), available at www.cdc.gov/abcs/index.html, as well as similar surveillance systems in Alaska and the Navajo Nation. Your local or state health department is your best source for guidance or additional information.
Pneumococcal disease is a serious disease that causes much sickness and death. Before the dip in pneumococcal disease observed in the first two years of the COVID-19 pandemic (when measures to control COVID-19 reduced the incidence of several infectious diseases), an estimated 30,000 cases and 3,000 deaths from invasive pneumococcal diseases (bacteremia and meningitis) occurred in the United States. Children younger than age two years and adults age 50 years and older have the highest incidence of serious disease. Case-fatality rates are highest for pneumococcal meningitis and bacteremia, and the highest mortality occurs among older adults and patients who have underlying medical conditions.