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The FDA approved a 21-valent pneumococcal conjugate vaccine (Capvaxive) for preventing invasive pneumococcal disease and pneumococcal pneumonia in adults, on Monday.

Specifically designed for an adult population, the single-dose vaccine is indicated for active immunization to prevent invasive disease and pneumonia caused by the following serotypes of the Streptococcus pneumoniae bacteria: 3, 6A, 7F, 8, 9N, 10A, 11A, 12F, 15A, 15C, 16F, 17F, 19A, 20A, 22F, 23A, 23B, 24F, 31, 33F, and 35B; and also for the prevention of invasive disease caused by serotype 15B.

Non-invasive cases of the bacterial infection include pneumococcal pneumonia, which is estimated to cause over 150,000 adult hospitalizations annually in the U.S., while invasive illness can include pneumococcal bacteremia, bacteremic pneumococcal pneumonia, and pneumococcal meningitis.

"Complications from invasive pneumococcal disease can lead to hospitalization, organ damage, and even death. Many cases of adult disease are caused by serotypes not included in other approved pneumococcal conjugate vaccines," Walter Orenstein, MD, of Emory University in Atlanta, said in a statement.

"Capvaxive is designed to include the serotypes that cause the majority of invasive pneumococcal disease in adults, helping to protect adults against invasive pneumococcal disease and pneumococcal pneumonia," added Orenstein, who is also a member of Merck's Scientific Advisory Committee.

The vaccine contains eight unique serotypes that are not covered by other available vaccines (15A, 15C, 16F, 23A, 23B, 24F, 31, and 35B).

Based on epidemiologic data from the CDC, the serotypes included in the vaccine together cause roughly 84% of the cases of invasive pneumococcal disease in adults 50 and older. By comparison, the 20-valent pneumococcal conjugate vaccine (PCV20) covers the serotypes responsible for 52% of cases in this age group.

Of note, the indication for the prevention of pneumonia was granted under the accelerated approval pathway based on immune response data and may be contingent on demonstration of clinical benefit in a confirmatory trial.

The CDC's Advisory Committee on Immunization Practices is to make recommendations on use of the new vaccine.

Approval was supported by four phase III trials -- including , , and -- that tested the vaccine in various adult populations, including in pneumococcal vaccine-naive and -experienced individuals, either alone or in combination with quadrivalent influenza vaccination.

The pivotal STRIDE-3 trial involved adults naive to a pneumococcal conjugate vaccine. The study demonstrated that the 21-valent vaccine was non-inferior to PCV20 in adults 50 and older for the shared serotypes -- as assessed by serotype-specific opsonophagocytic activity at 1 month post-vaccination -- and superiority for most of the serotypes unique to the 21-valent vaccine (immune responses were observed against serotype 15C, but not at levels meeting criteria for statistical significance). Immunobridging data showed that responses for individuals ages 18 to 49 were comparable to those in people 50 to 64 years.

The vaccine had a safety profile that was comparable to other available vaccines, including PCV20, the 15-valent pneumococcal vaccine (PCV15), and the pneumococcal polysaccharide vaccine (PPSV23), said Merck.

Commonly reported solicited adverse reactions in individuals 18 to 49 years who received the 21-valent vaccine included pain (73%), erythema (14%), and swelling (13%) at the injection site; along with fatigue (36%); headache (28%); and myalgia (16%). In adults 50 and over, reactions included pain at the injection site (41%), fatigue (20%), and headache (11%).

The 21-valent vaccine should not be administered to individuals with a history of a severe allergic reaction to any of its components or to diphtheria toxoid.

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