Michael Pride

An adequately powered, phase 3, pivotal efficacy trial is the strongest evidence of effectiveness to support vaccine licensure, but efficacy in lower powered phase 2 efficacy trials may be useful to justify progression to a costly and demanding phase 3 program. Conceptually, the design of an efficacy study is clear; an at-risk population is randomized to receive active vaccine or a control vaccine and systematically followed for cases of the target disease using a specific definition, usually combining a microbiological diagnosis and clinical features of the disease. Prior estimation of the expected number of cases in the control group is important in designing an efficacy study. The microbiological diagnostic method may be different than diagnostics in clinical practice and must be fully validated for a phase 3 regulatory efficacy trial. Flexible statistical design will allow interim analyses before the final analysis. This chapter will discuss key practical issues found across most vaccine efficacy trials, particularly regarding microbiological diagnosis and statistical methodology.

Clostridioides difficile ( C. difficile ) is a gram-positive, spore-forming bacterium that causes Clostridioides difficile infection (CDI). CDI manifests as significant gastrointestinal disease and is a major public health concern, particularly in vulnerable populations. CDI incidence has shown variable trends globally, with healthcare-associated infections declining due to improved infection control, but community-associated cases rising. CDI is driven by toxin production following gut microbiome disruption, often due to antibiotic use. Other risk factors for CDI include age, comorbid conditions, and immunosuppression. Despite advances in diagnostics, CDI underreporting remains significant due to inconsistent surveillance. Treatment strategies, including fidaxomicin, vancomycin, and fecal microbiota transplantation, aim to mitigate recurrence. Yet, protection against primary CDI remains elusive. Rigorous epidemiological studies and public health initiatives are critical to address CDI effectively. Advances in testing methods to detect both C. difficile and C. difficile toxins have improved diagnostic sensitivity and specificity. However, challenges persist in differentiating true infection from asymptomatic colonization. Clinical trials investigating CDI vaccines, such as those by Sanofi and Pfizer, have demonstrated strong immunogenicity but failed to meet primary efficacy endpoints for reducing first episodes of CDI incidence. These findings underscore the importance of continuing to develop vaccines to prevent CDI, as well as the challenges that are faced in demonstrating efficacy. The selection of appropriate endpoints for future studies based on the experience from both Sanofi and Pfizer in their phase 3 trials is especially critical.

It is sometimes impractical or unnecessary to conduct a timely efficacy study for a new vaccine. In these circumstances, it may still be possible to gain regulatory approval with a pivotal phase 3 study demonstrating effectiveness with an immunogenicity endpoint. For combination or line extension vaccines with components already known to be protective, immunogenicity may allow full licensure. For novel vaccines, an immunogenicity endpoint may be adequate evidence of effectiveness if it “is reasonably likely, based on epidemiologic, therapeutic, pathophysiologic, or other evidence, to predict clinical benefit,” allowing Accelerated Approval by the US Food and Drug Administration (FDA) (21 CFR 314.510-560). Such evidence may include protection by an approved vaccine, by naturally occurring protective antibodies or by evidence of protection by the vaccine in an animal model. Accelerated approval should not be considered an “easy” route to approval for several reasons. Rigorous justification is required for not performing an efficacy study, for endpoint selection and its relevance to predicted effectiveness, and for statistical methodology including non-inferiority margin and participant numbers. Assays must be stringently validated. An adequate clinical safety database must be available from preapproval trials. Subsequent demonstration of vaccine effectiveness is required for a novel vaccine, using evidence from a clinical study, generally but not always a postmarketing efficacy trial. Somewhat complementary, the “Animal Rule” (21 CFR 314.610-650 and 601.90-95) allows approval based on animal survival where a trial would be impossible or unethical. This would usually require an immunogenicity approach to bridge animal survival to predicted human survival. This route is also not an “easy” option, for many of the same reasons. Other regulatory agencies outside the USA may consider the same justifications for approvals based on immunogenicity.

Background: The 20-valent pneumococcal conjugate vaccine (PCV20) was developed to extend pneumococcal disease protection beyond 13-valent PCV (PCV13). Methods: This phase 3, double-blind study conducted in the United States/Puerto Rico evaluated PCV20 safety and immunogenicity. Healthy infants were randomized to receive a 4-dose series of PCV20 or PCV13 at 2, 4, 6 and 12–15 months old. Objectives included demonstrating noninferiority (NI) of PCV20 to PCV13 immunoglobulin G (IgG) geometric mean concentrations after doses 3 and 4 and percentages of participants with predefined IgG concentrations after dose 3, with 7 additional PCV20 serotypes compared with the lowest result among vaccine serotypes in the PCV13 group. Safety assessments included local reactions, systemic events, adverse events, serious adverse events and newly diagnosed chronic medical conditions. Results: Overall, 1991 participants were vaccinated (PCV20, n = 1001; PCV13, n = 990). For IgG geometric mean concentrations 1 month after both doses 3 and 4, all 20 serotypes met NI criteria (geometric mean ratio lower 2-sided 95% confidence interval > 0.5). For percentages of participants with predefined IgG concentrations after dose 3, NI (percentage differences lower 2-sided 95% confidence interval > –10%) was met for 8/13 matched serotypes and 6/7 additional serotypes; 4 serotypes missed the statistical NI criterion by small margins. PCV20 also elicited functional and boosting responses to all 20 serotypes. The safety profile of PCV20 was similar to PCV13. Conclusion: A 4-dose series of PVC20 was well tolerated and elicited robust serotype-specific immune responses expected to help protect infants and young children against pneumococcal disease due to the 20 vaccine serotypes. Clinical trial registration: NCT04382326.

To contain the coronavirus disease 2019 (COVID-19) pandemic, a safe and effective vaccine against the new severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) is urgently needed in quantities sufficient to immunise large populations. In this study, we report the design, preclinical development, immunogenicity and anti-viral protective effect in rhesus macaques of the BNT162b2 vaccine candidate. BNT162b2 contains an LNP-formulated nucleoside-modified mRNA that encodes the spike glycoprotein captured in its prefusion conformation. After expression of the BNT162b2 coding sequence in cells, approximately 20% of the spike molecules are in the one-RBD ‘up’, two-RBD ‘down’ state. Immunisation of mice with a single dose of BNT162b2 induced dose level-dependent increases in pseudovirus neutralisation titers. Prime-boost vaccination of rhesus macaques elicited authentic SARS-CoV-2 neutralising geometric mean titers 10.2 to 18.0 times that of a SARS-CoV-2 convalescent human serum panel. BNT162b2 generated strong T _H 1 type CD4 ⁺ and IFNγ ⁺ CD8 ⁺ T-cell responses in mice and rhesus macaques. The BNT162b2 vaccine candidate fully protected the lungs of immunised rhesus macaques from infectious SARS-CoV-2 challenge. BNT162b2 is currently being evaluated in a global, pivotal Phase 2/3 trial ( NCT04368728 ).

Identifying Streptococcus pneumoniae serotypes by urinary antigen detection (UAD) assay is the most sensitive way to evaluate the epidemiology of nonbacteremic community-acquired pneumonia (CAP). We first described a UAD assay to detect the S. pneumoniae serotypes 1, 3, 4, 5, 6A, 6B, 7F, 9V, 14, 18C, 19A, 19F, and 23F, covered by the licensed 13-valent S. pneumoniae conjugate vaccine. To assess the substantial remaining pneumococcal disease burden after introduction of several pneumococcal vaccines, a UAD-2 assay was developed to detect 11 additional serotypes (2, 8, 9N, 10A, 11A, 12F, 15B, 17F, 20, 22F, and 33F) in individuals with radiographically confirmed CAP. The specificity of the UAD-2 assay was achieved by capturing pneumococcal polysaccharides with serotype-specific monoclonal antibodies, using Luminex technology. Assay qualification was used to assess accuracy, precision, and sample linearity. Serotype positivity was based on cutoffs determined by nonparametric statistical evaluation of urine samples from individuals without pneumococcal disease. The sensitivity and specificity of the positivity cutoffs were assessed in a clinical validation, using urine samples obtained from a large study that measured the proportion of radiographically confirmed CAP caused by S. pneumoniae serotypes in hospitalized US adults. The UAD-2 assay was shown to be specific and reproducible. Clinical validation demonstrated assay sensitivity and specificity of 92.2% and 95.9% against a reference standard of bacteremic pneumonia. In addition, the UAD-2 assay identified a S. pneumoniae serotype in 3.72% of nonbacteremic CAP cases obtained from hospitalized US adults. When combined with bacteremic CAP cases, the proportion of pneumonias with a UAD-2 serotype was 4.33%. The qualified/clinically validated UAD-2 method has applicability in understanding the epidemiology of nonbacteremic S. pneumoniae CAP and for assessing the efficacy of future pneumococcal conjugate vaccines that are under development.