Groundbreaking Human Challenge Trials Propel RSV and hMPV Research

Respiratory syncytial virus (RSV) and human metapneumovirus (hMPV) pose significant health risks globally, particularly for infants, older adults, and individuals with underlying health conditions. Recent advancements in vaccine development highlight progress, but ongoing challenges remain in creating next-generation vaccines and antivirals. Traditional field trials often struggle with unpredictable seasonal transmission and lengthy study durations, prompting researchers to explore human challenge trials as a viable alternative.

hVIVO, a leader in human viral challenge studies, has developed the world’s first RSV B human challenge model, expanding upon its established expertise in the field. This innovation aims to enhance understanding of infection dynamics and immune responses while testing candidate vaccines and therapeutics more efficiently.

Innovative Approaches to Vaccine Development

Dr. Andrew Catchpole, Chief Scientific Officer at hVIVO, explains that RSV’s seasonal and unpredictable nature complicates traditional field trials. “You are dependent on natural exposure, which means you need very large sample sizes and long study durations to capture enough cases,” he notes. The introduction of human challenge trials addresses these limitations by offering a controlled environment where researchers can study infection patterns and immune responses with precision.

Most first-generation RSV vaccines relied on the A/Memphis strain for testing. As the field evolves toward bivalent and trivalent vaccines that target multiple strains, including RSV A, RSV B, and hMPV, the need for a robust RSV B model becomes critical. Dr. Catchpole emphasizes that developing an RSV B strain model allows for comprehensive testing of each vaccine component, ensuring broad protection against circulating strains.

The creation of this model begins with collecting clinical isolates, often sourced through a “friends and family” approach, where staff members gather samples from household contacts exhibiting respiratory symptoms. These samples are screened against a wide pathogen panel to identify promising RSV B candidates. Once identified, the isolates undergo rigorous testing for growth on GMP-compliant cell lines and extensive safety assessments, including targeted PCR panels and next-generation sequencing.

The resulting challenge virus exhibits high infection rates of approximately 90% by PCR criteria, alongside significant symptomatic disease. This is crucial, as it reflects the severity of cases typically encountered in field trials. When compared to the established RSV A Memphis model, the RSV B strain demonstrated higher levels of disease across several key endpoints, bolstering confidence in its effectiveness for evaluating vaccines and antivirals.

Regulatory Support and Future Prospects

The increasing receptiveness of regulators to challenge data derived from well-validated models marks a pivotal shift in the landscape of vaccine development. Several RSV vaccine candidates have received expedited designations, including Fast Track and Breakthrough designations, partially based on challenge study outcomes. This regulatory support accelerates the review process necessary for product development.

Challenge trials serve multiple purposes, assisting in dose selection, endpoint identification, and providing early efficacy signals that facilitate progress to larger field trials. While not a substitute for Phase III studies, these trials significantly enhance the development process. For instance, they enable researchers to evaluate individual components of combination vaccines, such as RSV A, RSV B, and hMPV in a trivalent formulation.

Dr. Catchpole highlights that challenge trials also offer a unique opportunity for optimizing antiviral treatment timing and dosage. Traditionally, treatment initiation occurs upon PCR positivity, which may not accurately represent real-world patient behavior. By incorporating patient perception triggers—dosing when volunteers feel symptomatic—researchers can create data that closely mirrors clinical practice.

As hMPV gains recognition as a significant pathogen, validated challenge models for each component of combination vaccines become essential. The newly established hMPV model demonstrates strong infection rates and symptoms comparable to those of RSV, forming a comprehensive platform alongside the RSV A and B models for evaluating future vaccines and therapeutics.

Dr. Catchpole anticipates a broader application of challenge trials in addressing early-phase research questions that are difficult to manage in traditional field studies. These trials can investigate correlates of protection, mucosal immunity, durability, and mechanisms of action, ultimately enhancing the development of complex respiratory vaccines.

Looking ahead, the combination of well-characterized respiratory virus challenge models is poised to serve as a cost-effective and efficient alternative to traditional Phase II studies. The introduction of the RSV B challenge model, along with advancements in hMPV modeling, signifies a significant leap forward in respiratory virus research.

As the global burden of respiratory pathogens persists, the acceleration of vaccine and antiviral development through human challenge trials may lead to meaningful improvements in public health outcomes. Dr. Catchpole and his team at hVIVO continue to push the boundaries of research, preparing to showcase their findings at the RSVVW’26 Conference in Rome, where they will discuss how their challenge models are shaping the future of respiratory virus development.