PCV21 Effective in Children, Adolescents at Increased Pneumococcal Disease Risk

Investigators at the 6th European Society of Clinical Microbiology and Infectious Diseases Conference on Vaccines in Lisbon, Portugal, have unveiled positive results from the phase 3 STRIDE-13 clinical trial (NCT06177912), in which the 21-valent pneumococcal conjugate vaccine (PCV21, Capvaxive; Merck) administered to high-risk children and adolescents aged 2 to less than 18 years who had previously completed a pneumococcal vaccine series demonstrated robust immune responses and noninferiority to the pneumococcal 23-valent polysaccharide vaccine (PPSV23), according to a news release from Merck.^1,2

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PCV21 was specifically designed to target serotypes causing most of the invasive pneumococcal disease (IPD) cases in adults, according to Paula Annunziato, senior vice president of infectious diseases and vaccines at Merck Research Laboratories. It is indicated for the prevention of invasive disease and pneumonia caused by Streptococcus pneumoniae serotypes 3, 6A, 7F, 8, 9N, 10A, 11A, 12F, 15A, 15B, 15C, 16F, 17F, 19A, 20A, 22F, 23A, 23B, 24F, 31, 33F, and 35B in individuals aged 18 and older. Although the STRIDE family of trials has affirmed PCV21’s effectiveness in adults, its potential in children had remained unknown until now.¹

“While [PCV21] was designed to specifically cover the serotypes that cause the majority of IPD cases in adults, findings from STRIDE-13 underscore its added potential to help protect children and adolescents who are at an increased risk,” Annunziato said in the news release.¹

Results of STRIDE-13 Show Immunogenicity in Younger Individuals

In STRIDE-13, a phase 3, randomized, double-blind, active comparator-controlled clinical trial, investigators evaluated the immunogenicity, safety, and tolerability of PCV21 compared with PPSV23 specifically in young patients with increased pneumococcal disease risk. A series of medical conditions can put patients at heightened risk for IPD, including diabetes, chronic liver disease, chronic lung disease, chronic heart disease, or chronic kidney disease. In total, 882 participants with these conditions were randomly assigned 3:2 to receive either a single dose of PCV21 or PPSV23 following completion of a primary pediatric pneumococcal vaccine regime. The pediatric regimen included either pneumococcal 7-valent conjugate vaccine, pneumococcal 10-valent conjugate vaccine, or pneumococcal 13-valent conjugate vaccine.¹

Thirty days post-vaccination, the immunogenicity of PCV21 serotypes was assessed through the measurement of serotype-specific opsonophagocytic activity (OPAs) geometric mean titers (GMTs). Simultaneously, safety was evaluated, measured as a proportion of individuals with adverse events (AEs).¹

The investigators found that PCV21 was immunogenic for all 21 serotypes included in the vaccine at the 30-day follow-up. Furthermore, immune responses elicited by the vaccine were noninferior to PPSV23 for each of their shared 12 serotypes, as determined by prespecific statistical criteria. For the 9 serotypes included in PCV21 but not PPSV23, PCV21 demonstrated superiority, as determined by serotype-specific OPA GMTs at 30 days.¹

Regarding safety, there was a generally comparable proportion of participants with solicited, systemic AEs and vaccine-related AEs; however, solicited injection-site AEs were observed more frequently in patients receiving PCV21 (72.3%) compared with PPSV23 (58.2%).¹

These new data signify the effectiveness of PCV21 in children and adolescents, highlighting its potential in this population. Previous studies have investigated PCV21 among infants and toddlers being coadministered other common pediatric vaccines, but these studies have included healthy individuals, while STRIDE-13 features patients who are at high risk for severe pneumococcal disease. The positive results could lead to future recommendations from regulatory bodies allowing younger individuals to receive PCV21.^1,3

“Children and adolescents living with chronic medical conditions are at increased risk of pneumococcal disease, and offering them additional protection is essential,” Rotem Lapidot, chief of pediatric infectious diseases at Rambam Health Care Campus and STRIDE-13 investigator, said in the news release. “Results from STRIDE-13 demonstrate the potential of [PCV21] 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.”¹

REFERENCES

1. Merck. CAPVAXIVE (Pneumococcal 21-valent Conjugate Vaccine) Demonstrates Positive Immune Responses in Children and Adolescents at Increased Risk of Pneumococcal Disease. News Release. Released September 11, 2025. Accessed September 12, 2025. https://www.merck.com/news/capvaxive-pneumococcal-21-valent-conjugate-vaccine-demonstrates-positive-immune-responses-in-children-and-adolescents-at-increased-risk-of-pneumococcal-disease/

2. A Clinical Study of the V116 Vaccine for Children and Teenagers (V116-013) (STRIDE-13). ClinicalTrials.gov Identifier: NCT06177912. Last Updated March 24, 2025. Accessed September 12, 2025.

3. Halpern L. PCV21 shows favorable immunogenicity in infants, toddlers receiving pediatric vaccines. Pharmacy Times. Published September 10, 2025. Accessed September 12, 2025. https://www.pharmacytimes.com/view/pcv21-shows-favorable-immunogenicity-in-infants-toddlers-receiving-pediatric-vaccines