Merck, known as MSD outside of the United States and Canada, announced significant new findings from its Phase 3 STRIDE-13 clinical trial of CAPVAXIVE® (pneumococcal 21-valent conjugate vaccine). The results, presented at the 6th European Society of Clinical Microbiology and Infectious Diseases (ESCMID) Conference on Vaccines in Lisbon, Portugal, demonstrate that the vaccine generated robust immune responses and a favorable safety profile in children and adolescents aged 2 to under 18 years who live with chronic medical conditions that increase their vulnerability to pneumococcal disease.
The Burden of Pneumococcal Disease
Streptococcus pneumoniae remains one of the most common bacterial pathogens worldwide. It is responsible for a wide range of illnesses, including pneumonia, meningitis, and bloodstream infections, collectively known as invasive pneumococcal disease (IPD). While routine pediatric vaccination has dramatically reduced the global burden of pneumococcal disease, certain populations continue to face disproportionate risk.
Children and adolescents with chronic health conditions such as diabetes, chronic lung disease, congenital heart disease, liver disease, or kidney disease are more susceptible to IPD. These conditions compromise the immune system’s ability to fight infections, making prevention through vaccination especially critical. Despite advances in pediatric vaccines, gaps in protection persist due to limited coverage of pneumococcal serotypes in existing immunization schedules.
Introducing CAPVAXIVE
CAPVAXIVE is a 21-valent pneumococcal conjugate vaccine, designed to broaden serotype coverage beyond existing conjugate vaccines. It targets 21 different pneumococcal serotypes, including nine not covered by the widely used PPSV23 (23-valent polysaccharide vaccine). The inclusion of these additional serotypes aims to close protection gaps, particularly in vulnerable populations such as older adults and high-risk children.
CAPVAXIVE is already approved in the United States, the European Union, Japan, and several other countries for use in adults aged 18 and older. Its development for pediatric and adolescent populations represents an important step in extending its benefits to younger individuals who face increased susceptibility to severe pneumococcal infections.
The STRIDE-13 Trial
STRIDE-13 (NCT06177912) was a randomized, double-blind, active comparator-controlled Phase 3 clinical trial. The study enrolled 882 participants aged 2 to under 18 years with one or more chronic conditions predisposing them to pneumococcal disease. Participants had previously completed a primary pediatric pneumococcal vaccination regimen, which could include PCV7, PCV10, or PCV13.
The trial randomly assigned participants in a 3:2 ratio to receive a single dose of either CAPVAXIVE or PPSV23. Researchers evaluated three main outcomes:
- Immunogenicity – measured through serotype-specific opsonophagocytic activity (OPA) geometric mean titers (GMTs) 30 days post-vaccination.
- Safety and tolerability – assessed by recording adverse events (AEs), including systemic and vaccine-related events.
- Comparative effectiveness – determining whether CAPVAXIVE was noninferior to PPSV23 for shared serotypes and superior for unique serotypes.
Key Findings
The results of STRIDE-13 were both statistically and clinically significant:
- Immune response across all 21 serotypes: CAPVAXIVE generated measurable OPA responses for all targeted serotypes 30 days after vaccination.
- Noninferiority to PPSV23: For the 12 serotypes present in both vaccines, CAPVAXIVE met prespecified statistical criteria for noninferiority, ensuring it performs at least as well as PPSV23.
- Superiority for nine unique serotypes: For the nine serotypes included only in CAPVAXIVE, immune responses were significantly higher compared to PPSV23, reinforcing the value of its broader serotype coverage.
- Safety profile: The overall rates of adverse events, including systemic and serious vaccine-related events, were similar between CAPVAXIVE and PPSV23. Local reactions such as injection-site pain were somewhat more common in the CAPVAXIVE group (72.3%) compared to PPSV23 (58.2%), but these events were generally mild to moderate in severity.
Expert Perspectives
Dr. Rotem Lapidot, Chief of Pediatric Infectious Diseases at Rambam Health Care Campus and an investigator in STRIDE-13, emphasized the public health significance of these results:
“Children and adolescents living with chronic medical conditions are at increased risk of pneumococcal disease, and offering them additional protection is essential. Results from STRIDE-13 demonstrate the potential of CAPVAXIVE to deliver protection for these vulnerable populations, who may benefit from additional pneumococcal disease coverage by including serotypes not contained in other approved pneumococcal infant regimens.”
Dr. Paula Annunziato, Senior Vice President of Infectious Diseases and Vaccines at Merck Research Laboratories, highlighted how these findings align with the company’s broader vaccine strategy:
“While CAPVAXIVE was designed to specifically cover the serotypes that cause the majority of invasive pneumococcal disease cases in adults, findings from STRIDE-13 underscore its added potential to help protect children and adolescents who are at an increased risk. We are encouraged by the safety and immunogenicity data, which underpin our commitment to ensuring infants, children, and adults have access to protection against invasive pneumococcal disease.”
Implications for Public Health
CAPVAXIVE’s broader serotype coverage represents a meaningful advancement in pneumococcal prevention. According to national-level CDC data, the vaccine covers approximately 84% of invasive pneumococcal disease cases in adults over 50, compared to about 52% covered by PCV20 (pneumococcal 20-valent conjugate vaccine).
In children and adolescents aged 2 to 17 years with elevated risk, CAPVAXIVE has the potential to cover approximately 78% of IPD cases, including 11 unique serotypes that account for around 34% of cases. This expanded coverage could translate into fewer severe infections and hospitalizations, particularly among immunocompromised or chronically ill children.
It is important to note that these percentages are based on epidemiological data and immune response measures, not direct head-to-head efficacy studies between CAPVAXIVE and PCV20. Continued research and confirmatory clinical trials will be critical in further validating CAPVAXIVE’s clinical benefits.
Regulatory Pathway and Next Steps
The data from STRIDE-13 mark the final readout of Merck’s Phase 3 STRIDE clinical program. These findings will be shared with regulatory authorities worldwide to support expanded approval for pediatric and adolescent use.
In the United States, CAPVAXIVE is already indicated for active immunization in adults 18 and older against invasive disease and pneumonia caused by 21 pneumococcal serotypes. The prevention indication for pneumonia in adults was granted under accelerated approval based on immune response data, with continued approval contingent on confirmatory evidence of clinical benefit. A similar regulatory strategy may apply to pediatric indications as well.
About CAPVAXIVE
CAPVAXIVE is Merck’s 21-valent pneumococcal conjugate vaccine indicated for active immunization for the prevention of invasive disease and pneumonia in adults 18 years of age and older. CAPVAXIVE is specifically designed to help address Streptococcus pneumoniae serotypes predominantly responsible for adult invasive pneumococcal disease (IPD), including eight unique serotypes, 15A, 15C, 16F, 23A, 23B, 24F, 31 and 35B compared to other approved pneumococcal vaccines. CAPVAXIVE is administered as a single dose.
Selected Safety Information for CAPVAXIVE in the U.S.
Do not administer CAPVAXIVE to individuals with a history of a severe allergic reaction (eg, anaphylaxis) to any component of CAPVAXIVE or to diphtheria toxoid.
Individuals with altered immunocompetence, including those receiving immunosuppressive therapy, may have a reduced immune response to CAPVAXIVE.
The most commonly reported (>10%) solicited adverse reactions in individuals 18 through 49 years of age who received CAPVAXIVE were: injection-site pain (73.1%), fatigue (36.0%), headache (27.5%), myalgia (16.4%), injection-site erythema (13.8%), and injection-site swelling (13.3%).
The most commonly reported (>10%) solicited adverse reactions in individuals 50 years of age and older who received CAPVAXIVE were: injection-site pain (41.2%), fatigue (19.7%), and headache (11.0%).
Vaccination with CAPVAXIVE may not protect all vaccine recipients.
About Pneumococcal Disease
Pneumococcal disease is an infection caused by a bacteria called Streptococcus pneumoniae. There are about 100 different types (referred to as serotypes) of pneumococcal bacteria, which can affect adults differently than children. Pneumococcal disease can be invasive or non-invasive.
Non-invasive pneumococcal illnesses include pneumonia (when pneumococcal disease is confined to the lungs), whereas invasive pneumococcal illnesses include pneumococcal bacteremia (infection in the bloodstream), bacteremic pneumococcal pneumonia (pneumonia with bacteremia) and pneumococcal meningitis (infection of the coverings of the brain and spinal cord). Pneumococcal pneumonia is a type of bacterial pneumonia, which is the most common clinical presentation of pneumococcal disease in adults. It’s estimated that over 225,000 adults are hospitalized from pneumococcal pneumonia each year in the U.S.
About Merck
At Merck, known as MSD outside of the United States and Canada, we are unified around our purpose: We use the power of leading-edge science to save and improve lives around the world. For more than 130 years, we have brought hope to humanity through the development of important medicines and vaccines. W
e aspire to be the premier research-intensive biopharmaceutical company in the world – and today, we are at the forefront of research to deliver innovative health solutions that advance the prevention and treatment of diseases in people and animals. We foster a diverse and inclusive global workforce and operate responsibly every day to enable a safe, sustainable and healthy future for all people and communities. For more information, visit www.merck.com and connect with us on X (formerly Twitter), Facebook, Instagram, YouTube and LinkedIn.
