The Lancet Infectious Diseases January 28, 2021

Immunogenicity and safety of a novel ten-valent pneumococcal conjugate vaccine in healthy infants in The Gambia: a phase 3, randomised, double-blind, non-inferiority trial

Ed Clarke, Adedapo Bashorun, Ikechukwu Adigweme, et al.

Bottom Line

A pivotal phase 3 non-inferiority trial demonstrating that a novel, low-cost 10-valent pneumococcal conjugate vaccine (Pneumosil) offers comparable immunogenicity and safety to an established vaccine (Synflorix) in healthy infants in The Gambia.

Key Findings

1. In the SIIPL-PCV (Pneumosil) group, post-primary seroresponse rates (defined as an IgG level ≥0.35 µg/mL) measured at 18 weeks of age were ≥89% for all targeted vaccine serotypes, with the exception of serotype 6A (79%).

2. SIIPL-PCV successfully demonstrated non-inferiority to the comparator vaccine (PHiD-CV) in both serotype-specific IgG geometric mean concentrations (GMCs) and overall seroresponse rates.

3. The trial demonstrated lot-to-lot equivalence for SIIPL-PCV, with the 95% CI around the GMC ratio for pairwise lot comparisons remaining within the strict 0.5 to 2.0 equivalence margins for all ten serotypes.

4. Reactogenicity was primarily mild-to-moderate; the most common solicited adverse events in both groups were injection-site tenderness and fever, with no serious or severe vaccine-related adverse events reported in the SIIPL-PCV group.

Study Design

Design

RCT

Double-Blind

Sample

2,250

Patients

Duration

18 wk

Median

Setting

Single center, Gambia

Population Healthy, PCV-naive infants aged 6-8 weeks

Intervention 0.5 mL of SIIPL-PCV (Pneumosil, a novel 10-valent pneumococcal conjugate vaccine) administered intramuscularly at 6, 10, and 14 weeks of age

Comparator 0.5 mL of PHiD-CV (Synflorix, a licensed pneumococcal polysaccharide protein D-conjugate vaccine) administered intramuscularly at 6, 10, and 14 weeks of age

Outcome Serotype-specific IgG geometric mean concentrations (GMCs) and seroresponse rates (defined as an IgG level ≥0.35 μg/mL) assessed at 18 weeks of age (1 month following the third dose)

Study Limitations

• The trial used immunogenicity (IgG thresholds and opsonophagocytic activity) as a surrogate marker for clinical efficacy, meaning the direct effect on clinical pneumonia or invasive pneumococcal disease was inferred rather than measured directly.

• Conducted at a single center in The Gambia, which may slightly limit the generalizability of the exact geometric mean concentration profiles to populations with significantly different genetic, environmental, or epidemiological backgrounds.

• The evaluation of cross-protection against non-vaccine pneumococcal serotypes relies on subsequent post-licensure real-world surveillance rather than trial data.

Clinical Significance

This trial provided the pivotal clinical data that supported the WHO prequalification of Pneumosil (PCV10-SII). By providing a highly affordable, safe, and immunogenic alternative that targets the pneumococcal serotypes most prevalent in low- and middle-income countries, it significantly enhances the sustainability and global reach of pediatric pneumococcal immunization programs, particularly through Gavi, the Vaccine Alliance.

Historical Context

Streptococcus pneumoniae remains a leading cause of child mortality globally, driving severe pneumonia, meningitis, and sepsis predominantly in low-resource settings. While earlier pneumococcal conjugate vaccines (such as Prevnar 13 and Synflorix) drastically reduced disease burden in high-income nations, their complex manufacturing processes and high costs limited their accessibility in the developing world. In response, the Serum Institute of India, in collaboration with PATH, developed Pneumosil—a 10-valent vaccine engineered specifically for the serotype epidemiology of Africa and Asia, aiming to provide equivalent protection at a fraction of the cost.

Guided Discussion

High-yield insights from every perspective

Med Student

Medical Student

Why do infant immunization schedules use a pneumococcal conjugate vaccine (PCV) like Pneumosil rather than a pneumococcal polysaccharide vaccine (PPSV23)?

Key Response

Infants under 2 years of age have an immature immune system that cannot mount an effective T-cell-independent immune response to pure bacterial polysaccharides. Conjugating the pneumococcal polysaccharide to a carrier protein recruits T-helper cells, inducing a robust T-cell-dependent response, immunological memory, class switching to high-affinity IgG, and mucosal immunity, which is essential for individual protection and reducing nasopharyngeal carriage.

Resident

In evaluating a young child presenting with suspected bacterial meningitis in a region that has newly implemented widespread PCV10 vaccination, how might this public health intervention alter your differential diagnosis and interpretation of epidemiological risks?

Key Response

Widespread PCV immunization drastically reduces the incidence of vaccine-type Streptococcus pneumoniae. While S. pneumoniae remains a top etiology for meningitis, the clinician must be highly suspicious of 'serotype replacement' (disease caused by non-vaccine pneumococcal serotypes) and consider shifts in relative frequency toward other pathogens like Neisseria meningitidis or Haemophilus influenzae. Empiric broad-spectrum coverage remains necessary, but the epidemiological pre-test probability shifts significantly.

Fellow

The trial utilizes a non-inferiority design comparing Pneumosil to Synflorix using a predefined serotype-specific IgG threshold (e.g., 0.35 mcg/mL). What are the limitations of relying on this aggregate correlate of protection when predicting true clinical efficacy against invasive pneumococcal disease?

Key Response

The WHO-established threshold of 0.35 mcg/mL is an aggregate correlate of protection against invasive disease, but it is an oversimplification. Protective thresholds actually vary significantly by individual serotype. Furthermore, measuring IgG concentration does not capture functional antibody activity, such as opsonophagocytic activity (OPA). Non-inferiority in immunogenicity does not perfectly guarantee non-inferiority in clinical effectiveness, especially against non-bacteremic mucosal syndromes like acute otitis media or pneumonia.

Attending

Given the economic barriers to PCV implementation in low- and middle-income countries, how does demonstrating the clinical non-inferiority of a highly affordable vaccine like Pneumosil shift the focus of global pediatric public health?

Key Response

Cost has historically been the primary barrier to global PCV equity, leaving millions of infants unprotected. By validating a low-cost, WHO-prequalified alternative, the primary public health bottleneck shifts from simply procuring affordable vaccines to strengthening cold chain infrastructure, addressing vaccine hesitancy, and ensuring timely administration of multi-dose schedules in remote areas. This represents a practice-changing milestone where implementation science becomes as critical as vaccine development.

Scholarly Review

Critical appraisal through the lens of expert reviewers and guideline development

PhD

In non-inferiority vaccine trials like this one, the primary analysis often relies on the Per-Protocol (PP) population rather than the Intention-To-Treat (ITT) population. Why is the PP population preferred in this specific statistical context, and what are the methodological risks if the study experiences high attrition in a resource-limited setting?

Key Response

In superiority trials, ITT is preferred because it is conservative and biases toward the null (no difference). Conversely, in non-inferiority trials, including non-compliant patients (ITT) can artificially dilute differences, making the two groups look erroneously similar and inappropriately biasing toward declaring non-inferiority. Therefore, the PP population is the standard primary analysis. High attrition or protocol deviations can severely shrink the PP cohort, reducing statistical power and potentially invalidating the non-inferiority conclusion if the missing data is non-random.

Journal Editor

As a peer reviewer evaluating this manuscript, how would you critically appraise the authors' choice of Synflorix (a 10-valent vaccine) as the active comparator instead of a 13-valent vaccine (PCV13), considering the serotype epidemiology in West Africa?

Key Response

A rigorous editorial review must question the clinical relevance of the comparator. Comparing PCV10 to PCV10 is methodologically sound for establishing non-inferiority of shared serotypes. However, if the local epidemiology in West Africa has a high burden of serotypes like 19A or 3 (covered by PCV13 but not PCV10), proving non-inferiority to Synflorix might lack real-world public health impact. A tough reviewer would flag this to ensure the manuscript adequately addresses baseline local serotype prevalence and the rationale for the comparator choice.

Guideline Committee

How should WHO SAGE and national immunization technical advisory groups (NITAGs) incorporate Pneumosil into current pneumococcal vaccination guidelines, and does this trial provide sufficient evidence to recommend product interchangeability in a mixed vaccine schedule?

Key Response

Current WHO guidelines strongly recommend the inclusion of PCVs in all childhood immunization programs. This phase 3 trial provides high-quality evidence of immunologic non-inferiority for a complete homologous series, supporting WHO prequalification and integration of Pneumosil as a primary, cost-effective option. However, because this trial only evaluated a homologous schedule (Pneumosil only), it does not provide direct evidence for interchangeability. Guideline committees would likely update guidelines to include Pneumosil as a standard option but maintain the recommendation to use the same product for the entire series unless the initial product is unavailable.

Clinical Landscape

Noteworthy Related Trials

2000

NCKP PCV7 Efficacy Trial

n = 37,868 · NEJM

Tested

7-valent pneumococcal conjugate vaccine (PCV7)

Population

Healthy infants

Comparator

Meningococcal group C conjugate vaccine

Endpoint

Vaccine-serotype invasive pneumococcal disease (IPD)

Key result: PCV7 was highly efficacious (97.4%) in preventing invasive pneumococcal disease caused by vaccine serotypes in infants and young children.

2006

Gambia PCV9 Efficacy Trial

n = 17,437 · Lancet

Tested

9-valent pneumococcal conjugate vaccine (PCV9)

Population

Healthy infants in The Gambia

Comparator

Placebo

Endpoint

First episode of radiologically confirmed pneumonia

Key result: The PCV9 vaccine reduced the incidence of radiologically confirmed pneumonia by 37% and reduced overall child mortality by 16%.

2014

COMPAS Trial

n = 23,821 · Lancet Infect Dis

Tested

10-valent pneumococcal conjugate vaccine (PHiD-CV / Synflorix)

Population

Healthy infants in Latin America

Comparator

Hepatitis B and DTPa-HBV-IPV vaccines

Endpoint

First episode of likely bacterial community-acquired pneumonia (B-CAP)

Key result: The PCV10 vaccine demonstrated 22% efficacy against likely bacterial pneumonia and 100% efficacy against vaccine-serotype invasive pneumococcal disease.

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