Beyond the Envelope: Why Vaccinia Virus Challenges Viral Clearance

A structurally robust model virus is re-emerging as a stringent standard in viral clearance validation

Ensuring viral safety is one of the central challenges in biopharmaceutical manufacturing. Although viral contamination events are rare, their potential consequences for product quality, supply continuity, and ultimately patient safety are significant. For this reason, regulatory frameworks such as ICH Q5A(R2) require clear evidence that manufacturing processes can effectively remove or inactivate potential viral contaminants. Viral clearance studies are therefore a mandatory component of process validation, relying on scaled‑down models in which defined model viruses are intentionally introduced into process steps to quantify their reduction [1,2].

Moreover, viral clearance is fundamentally based on demonstrating the capacity of individual process steps to reduce viral load using well‑selected model viruses and quantitative endpoints such as log reduction factors [1]. The choice of these model viruses is therefore not trivial; it directly impacts how conservative and meaningful the final safety claim will be [2].

 Against this general background, the question arises which model viruses provide sufficiently conservative clearance validation; this is where vaccinia virus (VACV) gains importance. As a member of the Poxviridae family, vaccinia virus is a large, structurally complex, enveloped DNA virus with distinct physicochemical characteristics. Its virion architecture, which includes multiple infectious forms with different membrane compositions, contributes to a comparatively high level of structural robustness. This distinguishes vaccinia virus from many commonly used enveloped model viruses, such as bovine viral diarrhea virus (BVDV) [3], as well as from other viral systems widely applied in viral clearance studies, including murine leukemia virus (MuLV), pseudorabies virus (PRV), or human immunodeficiency virus (HIV), which generally exhibit lower structural complexity [6].

This robustness is particularly relevant when evaluating virus-inactivation approaches. While enveloped viruses are generally considered more susceptible to chemical inactivation than non‑enveloped viruses, because their lipid envelope - essential for host cell entry - is readily disrupted by detergents and solvents, comparative studies have demonstrated that significant variability exists within this group [4]. Vaccinia virus is among the more resistant enveloped viruses under certain treatment and detergent conditions, particularly in the context of solvent/detergent processes [3,5]. However, this resistance is strongly dependent on the specific solvent/detergent system used, with some formulations achieving rapid and complete inactivation, as demonstrated for detergent‑mediated virus inactivation across different bioprocess matrices [6].

This perspective is further supported by expert discussions in the regulatory and scientific community. During conference exchanges and official stakeholder interactions, it has been communicated by regulatory experts that vaccinia virus can be regarded as one of the more stable enveloped viruses in detergent‑based inactivation systems. Such statements underline its relevance as a stringent test system for evaluating viral clearance performance.

The importance of this characteristic is increasing in the current industrial landscape. With the ongoing replacement of traditional detergents such as Triton X‑100 due to regulatory and environmental concerns, biopharmaceutical manufacturers are actively evaluating alternative formulations. These changes necessitate renewed viral clearance validation, particularly for solvent/detergent steps. In this context, there is a clear trend toward requesting more robust model systems to ensure that new approaches meet or exceed historical safety margins. Vaccinia virus is therefore increasingly considered as a preferred model to challenge these updated processes, especially given its demonstrated relevance for orthopoxvirus inactivation [7].

At the same time, it is important to recognize that the vaccinia virus is not a single, uniform entity. Multiple strains exist, each with distinct biological and physicochemical properties. Among the most relevant in the context of viral clearance are the Western Reserve (WR) vaccinia virus strain and the closely related orthopoxvirus, rabbitpox virus (Utrecht strain), both of which are well characterized and widely used in virological research [3, 8].

Historically, vaccinia virus and related orthopoxviruses have played an important role in evaluating viral safety for plasma‑derived medicinal products. Studies investigating solvent/detergent treatment have shown that vaccinia virus can be effectively inactivated but may exhibit measurable resistance depending on the specific process parameters and detergent systems applied [3,5]. In addition to chemical inactivation, physical removal mechanisms also contribute to viral clearance. Due to its large particle size, vaccinia virus can be effectively removed by filtration processes, providing an additional layer of safety in downstream processing [9]. This combination of inactivation susceptibility and relative robustness makes vaccinia virus particularly suitable for defining safety margins. Publications on orthopoxviruses in plasma product safety evaluations have demonstrated that such viruses can serve as relevant indicators for assessing the effectiveness and reliability of viral inactivation steps [4,7].

Taken together, vaccinia virus represents a well‑established, scientifically robust model for viral clearance studies. Its structural complexity, relative stability within the class of enveloped viruses, and extensive characterization make it particularly valuable for modern process validation. As the biopharmaceutical industry continues to evolve, particularly with changes in detergent systems and increasing regulatory expectations, vaccinia virus is re-emerging as a key tool for generating conservative, reliable, and defensible viral safety data.

References:

1.    Ruppach H., Log10 Reduction Factors in Viral Clearance Studies. BioProcess Journal, 2014.
2.    Ruppach H., Viral Safety for Biotherapeutics and Biosimilars. Drug Discovery Today: Technologies, 2020.
3.    Remington KM, et al. Inactivation of West Nile virus, vaccinia virus and viral surrogates for relevant and emergent viral pathogens in plasma-derived products. Vox Sang., 2004.
4.    Dichtelmüller H, et al. Robustness of solvent/detergent treatment of plasma derivatives: a data collection from Plasma Protein Therapeutics Association member companies. Transfusion, 2009.
5.    Roberts P. Resistance of vaccinia virus to inactivation by solvent/detergent treatment of blood products. Biologicals, 2000.
6.    Farcet JB, et al-, Detergent-mediated virus inactivation in biotechnological matrices: More than just CMC. Int J Mol Sci, 2023.
7.    Kindermann J, et al., Orthopox viruses and the safety margins of solvent-detergent treated plasma-derived medicinal products. Transfusion, 2022.
8.    Adams et al., Rabbitpox Virus and Vaccinia Virus Infection of Rabbits as a Model for Human Smallpox. J Virol., 2007.
9.    Berting A et al., Effective poxvirus removal by sterile filtration during manufacture of plasma derivatives. J Med Virol., 2005.

Philipp Petermann, PhD, Head of Viral Clearance and Test Facility Manager (GLP) at the Charles River site in Cologne, Germany, joined Charles River in 2014. He initially worked as a Study Director and later as a Study Director Supervisor in the Viral Clearance department. Prior to joining Charles River, Philipp worked as a postdoctoral researcher at the University Hospital of Cologne, where his research focused on herpes simplex virus entry mechanisms in primary human and murine skin cells and tissues.