Challenges in vaccine transport: can we deliver without the cold chain?

KEYWORDS:

• cold chain
• film
• formulation
• freeze-drying
• manufacturing
• spray-drying
• thermostability
• vaccine

1. Introduction

In 2020, the world watched global cold chain and distribution networks crumble as demand for goods increased while operations became constricted. This intensified as the first COVID-19 vaccines became available. The requirement for storage at ultralow temperatures posed a notable barrier to rapidly distributing them throughout the US and significant delays in reaching low- and middle-income countries. It also illustrated how complex and fractured the global cold chain is today (Figure 1). These challenges were not new to vaccine manufacturers as economic loss from cold disruption is included in cost estimates for every vaccination campaign [1]. Prior to the COVID-19 pandemic, improvements in cold chain systems and development of transport containers to get vaccines into remote areas without spoiling have long been a priority for organizations like the WHO and PATH. More recently, programs led by CEPI, GAVI, BMGF, and BARDA, among others, have fostered a shift in the approach to look at the vaccines themselves and what else goes in the vial.

Figure 1. Overview of the current global cold chain distribution system for vaccines.

2. Degradation of vaccines

Common chemical and physical pathways by which vaccines degrade are summarized in Figure 2. These processes are initiated by light, product contaminants (peroxides, metals), and fluctuations in temperature and pH [2]. Physical agitation and changes in ionic strength and pH during freeze–thaw cycles facilitate the unfolding of antigens, exposing regions that serve as points of attachment for neighboring antigens to form large, insoluble particulates which compromise vaccine stability and performance [3]. Selection of the right combination of excipients and method of stabilization can protect antigens against physical and chemical stressors and extend the shelf life of a vaccine.

Figure 2. Chemical and physical pathways of vaccine degradation.

3. The current state

Of the 95 vaccines approved by the US FDA, ~70% are liquids stored at 2–8°C with expiration dates from 16 weeks to 3 years [4]. Many must be discarded if inadvertently frozen during transport or storage. Approximately 20% are freeze-dried powders. Most are stored at 2–8°C, and over half can withstand temporary exposure to freezing temperatures. Those listed as freeze-sensitive contain live, attenuated viruses, or virus antigens. Most lyophilized vaccines must be discarded if unused within 1–6 h after they are rehydrated for injection. Many vaccines provided as frozen liquid solutions must be discarded if they are accidentally thawed during transit. Vaccine manufacturers have begun to include some information about stability after temporary exposure to elevated temperatures where the cold chain can be the most fragmented. For example, the package insert of a lyophilized Yellow Fever vaccine (YF-VAX) states ‘Half-life is reduced from approximately 14 days at 35° to 37°C to 3–4.5 days at 45° to 47°C,’ while that of a liquid, frozen Ebola Vaccine (ERVEBO) states ‘A thawed vial can be stored refrigerated ….for no more than 2 weeks and at room temperature ….for no more than 4 hours.’

4. Stabilization approaches: liquid products

Sugars, polymers, surfactants, and amino acids have been used to develop safe and efficacious liquid vaccine products. Sugars interact with water to create a hydration shell that keeps an antigen in its native conformation [3]. Amino acids prevent aggregation by increasing the surface tension of water [5]. Surfactants form micelles that can trap antigens inside and/or form networks between individual antigens to prevent aggregation and loss of product due to binding to container surfaces [6]. Polymers compete with antigens at the air–water interface of liquid products and prevent aggregation through electrostatic interactions that force antigens into their native confirmation through molecular crowding [3]. Malfunctions in storage units, accidental freezing, and partial thawing of vaccines during global transportation and distribution are a common reason for loss of vaccine potency due to shredding of antigen structure during ice formation. Use of salts and sugars at very high concentrations can significantly reduce this effect; however, these solutions cannot be comfortably administered. Alum-based adjuvants partially prevent ice-crystal formation; however, if accidentally frozen, they can accelerate aggregation [7]. Diphtheria and tetanus vaccines have some of the best stability profiles for liquid vaccine products as they are stable for up to 2 years at 18°C if they remain unfrozen [7]. To improve the thermostability profiles of classical liquid vaccine formulations, pharmaceutical scientists have considered solid state stabilization technologies.

5. Stabilization approaches: solid products

Freeze-drying has been used to extend the shelf life of foods, small-molecule and biological drugs, and some vaccines [8]. Freeze-drying is a multi-step process. Freezing rate and selection of appropriate sugars and polymers must be optimized to minimize ice crystallization and damage during primary drying where water is removed through sublimation [3]. Additional water is removed to achieve an optimal residual moisture content of 1–3% for most vaccines during secondary drying where temperature is increased to ambient temperature at a controlled rate. Because antigenic structure can be disrupted during any stage of this process, specific diluents that contain surfactants prevent aggregation and adhesion of compromised antigens to container surfaces [6]. Freeze-drying processes can take up to 72 h to complete. A report of a freeze-dried influenza vaccine with favorable stability at 4°C, 25°C, and 37°C for 40 months demonstrates that understanding of the physical characteristics of viruses, excipient selection, and the freeze-drying process can produce more thermostable vaccines in the future [9].

Spray-drying, an alternative to lyophilization, requires identification of the appropriate combination of sugars, polymers, and surfactants that maintain the three-dimensional shape of antigens during atomization and drying at temperatures of 60–220°C [10]. This process has been modified to accommodate temperature sensitivities of vaccine antigens by lowering inlet air temperature and changing air-flow patterns to minimize exposure to high temperatures for less than 30 s. Reports describing robust immunogenicity of a live spray-dried tuberculosis vaccine and a recombinant HPV vaccine after 2 and 3 years respectively at room temperature hold promise for this technology in getting vaccines out of the cold chain [10].

Film-based dosage forms, traditionally used for delivery of small-molecule drugs by the sublingual or buccal route, have recently garnered attention for their potential to improve vaccine stability and deliver them in a needle-free manner. Conventionally, films are prepared by solvent casting and the use of heat and organic solvents to accelerate the manufacturing process. An appropriate combination of sugars, polymers, and surfactants in biologically compatible buffers with drying under constant airflow at ambient temperature stabilized live, recombinant adenovirus for 3 years at 20°C. This formulation also supported a strong anti-influenza immune response following sublingual administration to mice [11]. Film-based formulations have also been shown to be compatible with microneedle technology to improve the intradermal and buccal administration of vaccines [12].

6. Expert opinion

According to the CDC, 82.1 million COVID-19 vaccine doses were discarded in the US between December 2020 and June 2021. Using the current cost of $130 USD for the Pfizer COVID vaccine, this represents a loss of $1.1B, which correlates with the WHO estimates for any large-scale vaccination campaign [1]. While many attribute the waste to vaccine hesitancy, which was an issue globally [13], the center of the problem was the short expiration date of the vaccines, even if they were stored and transported properly at −80°C. If they had an expiration date of 3 years, for example, they would not be included in these waste calculations as they could remain in stock in pharmacies. Other primary reasons for waste include freezer/cold chain failures and doses left unused in open multi-dose vials, which could have been saved by identification and use of technologies that allow vaccines to be distributed and stored without the need for strict temperature monitoring.

Early vaccines were designed as liquid formulations and have saved countless lives. They can be rapidly prepared and are easy to administer; however, many must be stored at sub-zero temperatures and all require strict cold-chain maintenance due to temperature sensitivity of vaccine antigens [4]. Freeze-drying technologies have moved some vaccines out of the freezer; however, there are no currently approved lyophilized vaccines that can be stored at ambient temperatures [4]. The recent report of the stability of a tailor-made vaccine for Africa through the Meningitis Vaccine Project (MenAfriVac, 2 years at 25°C/70% RH, 6 months at 40°C/70% RH) demonstrates the advantage that freeze-drying can offer some vaccines [14]. However, the cost and complexity of this process and detailed reconstitution procedures make it an impractical form of stabilization, especially in pandemic situations. Spray-dried products have significantly lower operating costs than those that are freeze-dried and do not require reconstitution for administration as they are predominately administered by inhalation. Spray freeze drying, where the formulated vaccine is sprayed into liquid nitrogen and frozen droplets subsequently freeze-dried, has shown some promise in stabilizing vaccines but has not yet been shown to be scalable [2,10].

Within the next 10 years, we predict that there will be a massive shift toward alternative methods for stabilization of vaccines in response to cold chain concerns. Vaccines will get out of the freezer and no longer rely upon the cold chain. Technologies that can address this as well as other issues associated with lack of vaccine compliance, due to trypanophobia, will significantly improve public health. Film-based technologies ultimately offer the simplest, cost-effective solution for vaccine stabilization for patients, providers, manufacturers, and distributors of vaccines. Production of films under ambient conditions and direct administration to the oral mucosa or skin simplifies key challenges associated with freeze- and spray-dried products which have not yet been addressed by emerging 3-D printing technologies [15]. Film-based vaccines have also been shown to be resistant to freeze–thaw stress and can induce protective immune responses through needle-free administration [11]. This highlights the ability of film technology to overcome previous stability limitations and bypass needle administration, possibly improving access and compliance to vaccines worldwide. While there are currently few institutions experienced in stabilization of vaccines within a film matrix, significant work is underway in our laboratories to develop systems which allow large-scale manufacturing of films containing vaccines to accommodate childhood immunization campaigns and pandemic scenarios, alike.

Abbreviations

BARDA: Biomedical Advanced Research and Development Authority, BMGF: Bill and Melinda Gates Foundation, CDC: Centers for Disease Control, FDA: Food and Drug Administration, GAVI: Global Alliance for Vaccines and Immunization, PATH: Program for Appropriate Technology in Health, USD: United States Dollars, WHO: World Health Organization

Declaration of interest

M Croyle serves as co-founder of and scientific advisor for Jurata Thin Film and holds several patents on film-based stabilization technology. I Bajrovic and M Croyle hold equity in Jurata Thin Film. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or material discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Author contribution statement

I.B. and M.A.C. equally contributed to the conception and design of this publication and actively participated in writing and revising it for intellectual content.

Acknowledgments

The authors thank Stephen C. Schafer for assistance in the preparation of the figures presented in this manuscript.

Additional information

Funding

This work was supported by a Glaxo Wellcome Endowed Fellowship (MAC).