Influenza Study Design Parameters and Endpoints

Study Designs

Influenza research employs various study methodologies to evaluate prevention, treatment, and epidemiology:

• Randomized controlled trials comparing favipiravir (1800 mg BID on day 1, 800 mg BID on days 2-5) versus placebo in adults with uncomplicated influenza
• Cross-sectional studies for assessing vaccination coverage among healthcare personnel
• Prospective global study (HARTI) conducted in adults hospitalized with acute respiratory tract infection across 40 centers in 12 countries
• Phase IIIb-IV, multicenter, randomized, double-blind, active-controlled trials
• Double-blind, placebo-controlled trials
• Retrospective cohort studies
• Real-world studies using national health databases
• Quantitative analysis of media coverage on influenza and immunization-related topics
• Real-time RT-PCR methods for influenza virus detection (subtypes A(H1N1)pdm09, H3N2, and influenza-B)
• In vitro and in vivo testing of treatments such as equine polyclonal immunoglobulin fragments (F(ab')2)
• Suspension microbead array (MBA) analysis for detecting influenza-specific antibodies

Study Populations

Research has focused on diverse populations:

• Adults with uncomplicated influenza in favipiravir trials (N=827 for favipiravir, N=419 for placebo)
• Adults 65 years of age or older in high-dose vaccine trials
• Healthcare personnel in vaccination coverage studies
• Adults hospitalized with acute respiratory tract infections
• BALB/c mice challenged with H5N1 for testing treatments
• Patients with influenza-like-illness for cytokine studies (350 patients screened)
• Caregivers of hospitalized children (N=522; 88.9% response rate)
• Otherwise healthy adults with influenza-like symptoms and fever of ≤48 hours (favipiravir trials)
• High-risk outpatients aged ≥6 months with acute respiratory illness
• Medicare beneficiaries aged ≥65 years

Primary Endpoints

Key primary endpoints in influenza studies include:

• Time to illness alleviation when 6 influenza symptoms were self-rated as absent or mild and fever was absent (favipiravir trials)
• Laboratory-confirmed influenza caused by any viral type or subtype associated with protocol-defined influenza-like illness (high-dose vaccine trial)
• Neutralizing titers against H1N1 virus (subjects classified as responders if titers increased ≥4-fold at D14 compared to D0)

Secondary Endpoints and Efficacy Measurements

Studies evaluate multiple secondary outcomes:

• Viral titers in nasopharyngeal samples measured as log10TCID50/ml
• Viral titers and RNA load area under the curve over days 1-5
• Median times to cessation of virus detection
• Plasma concentrations of favipiravir with target average Cmin ≥20μg/ml
• Hemagglutination-inhibition (HAI) titers
• Seroprotection rates (percentage of participants with HAI titers ≥1:40)
• Antibody responses to influenza
• Prevention of influenza-associated illnesses, medical visits, hospitalizations, and deaths
• Relative vaccine effectiveness (RVE) in preventing hospital encounters
• Cytokine levels (IL-6, IL-8, IFN-γ, TNF-α, IL-10) measured by cytometric-bead-array with flow cytometry
• mRNA expression of cytokines quantified by real-time RT-PCR
• Vaccination coverage rates among different healthcare worker categories
• Survival rates in mice treated with F(ab')2 against H5N1 (60% increase with optimal protocol)
• Length of hospital stay and supplemental oxygen use in respiratory infections
• Readmission rates (20-33% within 3 months post-discharge)
• In-hospital mortality for RSV (2.5%), influenza (1.6%), and hMPV (2%)
• Influenza vaccine acceptance among caregivers of hospitalized children (30.3%)

Safety Assessments

Safety monitoring includes:

• Serious adverse events during safety surveillance period
• In high-dose vaccine trial: 8.3% of participants in IIV3-HD group vs 9.0% in IIV3-SD group reported at least one serious adverse event (relative risk, 0.92; 95% CI, 0.85-0.99)
• Asymptomatic hyperuricemia noted in favipiravir trials

Statistical Analyses

Analytical approaches include:

• Intention-to-treat analysis
• Relative efficacy calculations with 95% confidence intervals
• Poisson regression
• Inverse probability of treatment weighting (IPTW)
• Test-negative design for estimating vaccine effectiveness
• Compartmental model to estimate numbers of influenza-associated outcomes prevented by vaccination
• Multivariate analysis to identify independent risk factors
• Mann-Whitney nonparametric tests for comparing antibody responses
• Multivariable logistic regression to examine associations between survey responses and vaccination status
• Chi-square tests to compare vaccination coverage between different healthcare professions
• Post-hoc analyses to assess frequency of reaching target drug concentrations and association with antiviral efficacy

Biomarkers and Correlates

Studies utilize various biomarkers:

• Immunoglobulin levels (IgA1 and IgG1) as markers of immune response
• Cytokine profiles as indicators of disease severity
• T-705-M1 to favipiravir ratio as marker of drug metabolism