Subunit Vaccines and the Fight Against COVID-19

Advances in technology are accelerating the development and manufacture of subunit vaccines, an established class of vaccines.

As the name implies, subunit vaccines are based on a portion of the infectious agent rather than leveraging the entire virus (killed or live-attenuated). A subunit vaccine usually contains a protein, a polysaccharide, or a combination of both. Because they contain only a portion of the pathogen, subunit vaccines typically have fewer side effects and can be given to a wider group of people, including those with compromised immune systems and chronic health conditions. They typically lack pathogen-associated molecular patterns, however, and thus tend to cause weaker immune responses, requiring the use of adjuvants and possibly booster doses.

The first recombinant protein vaccine was developed for hepatitis B, and many more have been approved since. Subunit vaccines formulated as virus-like particles (VLPs) and nanoparticles are also under development today.

Direct response

Subunit vaccines work by exposing the body to a piece of a pathogen (virus, bacteria, parasite) that triggers an immune response, according to a spokesperson from Novavax. The company’s NVX-CoV2373 candidate vaccine against COVID-19, for example, uses an optimized version of the full-length spike (S) protein from SARS-CoV-2 as the subunit antigen. This antigen cannot cause disease but is delivered in a way that is read to be recognized and learned by the immune system.

In contrast, genetic vaccines (DNA or RNA vaccines) only take a small part of the viral genetic information that encodes the antigen triggering the immune response, notes Axel Erler, associate director of commercial development with Lonza. RNA vaccines, however, don’t generate a direct immune response, adds the Novavax spokesperson. Instead, they deliver a molecule that must be translated into the desired antigen, which is then recognized by the immune system.

Established production platform

One advantage of subunit vaccines is that their manufacture is achieved using established recombinant technology that is widely distributed in the biopharma world, according to Yves Balmer, head of microbial development services at Lonza. “This technology offers a well-defined technical and regulatory landscape that is readily available and does not require the culture of virulent organisms and their subsequent attenuation/inactivation, which can create safety concerns,” he says.

In addition, Balmer notes that the recombinant production of subunit vaccines enables the selection of specific antigens that can be combined in a multivalent vaccine and generate a well-characterized product. Furthermore, production platforms for subunit vaccines are highly adaptable, with exactly the same procedures used to develop and manufacture variant-strain versions for use in scenarios of genetic drift, according to a spokesperson from Novavax.

Highly stable vaccines

One key advantage of subunit vaccines over genetic vaccines is their strong stability profile. Transport and storage can take place at regular refrigeration temperatures, and there is no need for ultra-cold freezers. “This feature enables the distribution of subunit vaccines in diverse global locations where -20 °C or -80 °C cold chains are unavailable,” Novavax’s spokesperson observes.

Such an ability is not an insignificant factor. “It is critical nowadays to set up resilient supply chains to be prepared for any disruptions such as COVID-19. In addition to having transparent and scenario-based forecasting in place to anticipate risk-based future global demand scenarios, managing cold-chain requirements regarding storage and transportation remains a key capability, especially considering the variety of temperature classes (i.e., cool-chain to deep-frozen, all the way down to liquid nitrogen temperatures),” comments Christian Rochel, head of supply chain for biologics at Lonza’s Visp, Switzerland facility.

Rochel adds that a focus on internal and external supply capabilities, including multi-site inventory management, remains an important factor to ensure full supply.

Used in currently approved products

Unlike novel approaches and emerging technologies for vaccine development that can lead to regulatory challenges regarding demonstration of the safety and efficacy of such vaccine candidates, the development and regulatory pathways to commercialization for subunit vaccines are well established, according to Karen Magers, director of regulatory affairs at Lonza. “Vaccine regulators tend to be conservative, and there is a requirement for providing rigorous, supportive data based on the large intended populations for most vaccines, which often include children,” Magers says.

Protein subunit vaccines are an established technology, adds the Novavax spokesperson, and there are several approved subunit vaccine products on the market. Noteworthy for the company is the fact that a recombinant protein subunit vaccine manufactured using Sf9-derived insect cells, the approach used by Novavax for NVX-CoV2373, was approved in 2013. The investigational vaccine candidate being developed by Novavax, however, is also undergoing rigorous evaluation of its safety and efficacy, along with characterizing the company’s manufacturing processes.

One potential challenge for subunit vaccines is the need to use adjuvants in these formulations to achieve robust/broad immune responses. The issue arises, according to Magers, if a novel adjuvant is introduced. “Regulatory challenges can arise while establishing the safety profile of new adjuvants,” she remarks.

Development takes investment

The time and money needed to develop subunit vaccines can pose additional challenges. While development timelines for these vaccines are shorter than those for vaccines based on whole pathogens, subunit vaccine development is still time-consuming. One key hurdle is determining the right antigen. Determining the best dose and adjuvant combinations can also take time, according to the spokesperson from Novavax.

In addition, many specific subunits have unique physicochemical properties, and a tailored process needs to be developed for each, starting at the level of the expression host then moving to process and analytical method development, says Balmer. “This process development for non-platform molecules requires expertise, time, and flexibility in the manufacturing facility,” he adds.

The cost of goods for subunit vaccines can be an issue as well. “Recombinant protein technology was initially developed for biopharmaceutical drugs, for which the cost of producing the drug substance was not a critical factor,” Balmer observes. In contrast, vaccines are expected to be affordable considering the need for wide implementation.

An approach to address both issues, Balmer notes, is to develop platform processes that would shorten the development timeline, pre-define the manufacturing facility, and reduce the cost. “An example is the production of VLPs in which the frame remains constant and only the antigenic part is target-specific,” he says. This standardized approach, however, is only successful if the antigenic portion remains limited so it does not impact the production process.

For its COVID-19 vaccine candidate, Novavax had a head start due to its recent experience developing vaccines for the original severe acute respiratory syndrome virus and its NanoFlu candidate for seasonal influenza, according to the company spokesperson. This experience combined with knowledge of the coronavirus’ spike protein and access to the company’s nanoparticle technology and proprietary Matrix-M adjuvant has helped to accelerate development ofNVX-CoV2373. Today, Novavax can begin testing a new vaccine candidate in the pre-clinical environment within weeks of identification of a variant of concern.

Many advances in the last decade

The concept of subunit vaccines has been around for decades. Several advances in technology have helped improve their development and manufacture. Magers points to progress in genomics for the identification of vaccine candidates and incorporation of three‐dimensional (3D) structure, domain organization, and dynamics of surface proteins analysis into vaccine design as aiding development efforts.

There have also been improvements in subunit vaccine formulation, according to Magers, such as the use of nanoparticles (e.g., ferritin) that self-assemble into microscopic particles that display a protein antigen and the introduction of novel adjuvants (e.g., CpG 1018 used in Heplisav-B, AS04 [monophosphoryl lipid A, MPL] used in Cervarix, and AS01B [MPL and QS-21] used in Shingrix).

Manufacturing advances of note for Magers include expanding use of different expression systems including mammalian, insect, microbial, and fungal cell lines; incorporation of single-use technologies and equipment and closed systems into manufacturing processes; exploration of continuous manufacturing and quality-by-design approaches; and the introduction of novel improved analytical methods (e.g., mass spectrometry, particle analysis methods, and capillary electrophoresis) in conjunction with an emphasis on replacing in-vivo potency assays.

Since the first subunit vaccine was approved for hepatitis B, Novavax has advanced the technology for this class of vaccines through its use of a nanoparticle core to present the protein subunits to the immune system in a way that results in robust, durable responses that offer protection in the face of genetic drift, according to the company’s spokesperson. In addition, evaluation of Novavax’s vaccine technology has also revealed additional uses for its proprietary Matrix-M adjuvant when paired with vaccine antigens developed by other organizations.

Combination subunit vaccines are also now being explored, such as Novavax’s NanoFlu + NVX-CoV2373 candidate, to address multiple infectious disease threats. This investigational combination subunit vaccine, which is made possible by technology that is flexible, uses very low amounts of antigen, and is combined with an adjuvant that shows an encouraging safety profile, has already shown strong results in pre-clinical evaluations, according to the Novavax spokesperson.

Role to play in the fight against COVID-19

More than 20 protein subunit vaccine candidates have entered clinical trials for COVID-19, including those from Novavax, Anhui Zhifei Longcom Biopharmaceutical, Kentucky Bioprocessing, and Sanofi/GlaxoSmithKline, among others. More than 50 other candidates are at the preclinical stage.

“We believe that whether by providing best-in-class efficacy as a primary immunization series or boosting those previously vaccinated with our or another vaccine, subunit vaccines promise to be a vital part of the global effort to fight the COVID-19 pandemic. With a cold chain that does not require freezing, these vaccines will likely be delivered and used around the world,” states Novavax’s spokesperson.

In the Novavax NVX-CoV2373 candidate, the spike protein is organized around a nanoparticle and formulated with Matrix-M adjuvant. This technology platform using the full-length spike protein with Matrix-M adjuvant has, according to Novavax’s spokesperson, delivered outstanding efficacy against the original COVID-19 virus (96.4% in a United Kingdom Phase III clinical trial), protection against variants, and a favorable safety profile. In mid-June 2021, the company reported 90.4% efficacy overall from a Phase III trial in the US and Mexico.

“Organizing spike proteins around a nanoparticle core enables the immune system to learn different facets (epitopes) of the spike protein, including cryptic/hidden epitopes, which we think helps to explain the strong clinical results from our Phase I, II, and III studies,” Novavax’s spokesperson adds.

In addition, Novavax has established a global manufacturing network to increase its capacity to respond to infectious disease threats, including COVID-19. In announcing the latest clinical trial results, the company said it plans to file regulatory authorizations in the third quarter of 2021 after it completes final phases of process qualification and assay validation needed to meet chemistry, manufacturing, and controls requirements (1).

1. Novavax, “Novavax COVID-19 Vaccine Demonstrates 90% Overall Efficacy and 100% Protection Against Moderate and Severe Disease in PREVENT-19 Phase 3 Trial,” Press Release, June 14, 2021.