Inside the rigorous, step-by-step process the CDC uses to create pandemic-ready flu vaccine viruses, long before the next global health emergency strikes.
Study: Development of pre-pandemic influenza candidate vaccine viruses for use in vaccine manufacturing. Image Credit: Pickadook / Shutterstock.com
In a recent paper published in npj Vaccines, researchers from the United States Centers for Disease Control and Prevention (CDC) outline the comprehensive methodology used to develop influenza candidate vaccine viruses (CVVs). By adhering to strict Quality System Requirements (QSR) and World Health Organization (WHO) guidelines, this protocol supports the rapid transition from CVV development to vaccine manufacturing, helping ensure that pandemic influenza vaccines can be produced efficiently when needed.
Maintaining flu vaccine stockpiles
Influenza remains one of the most prevalent pathogens worldwide, with seasonal flu epidemics responsible for up to 650,000 deaths annually. Consequently, a novel pandemic strain could be catastrophic, particularly in a public state of unpreparedness, as demonstrated by the coronavirus disease 2019 (COVID-19) pandemic.
Widespread vaccination coverage remains the most effective strategy to protect populations against the devastating impacts of infectious outbreaks. Currently, the CDC, in partnership with domestic and international entities like the U.S. National Institutes of Health and WHO, develops and maintains candidate vaccine viruses, while U.S. government preparedness programs such as the Biomedical Advanced Research and Development Authority (BARDA) support vaccine stockpiling efforts against influenza A lineages with high pandemic potential to ensure their availability in the event of a pandemic.
CVVs developed at the CDC are optimized for efficient growth in chicken eggs, serving as primary vaccine seeds for mass production.
How are CVVs produced by the CDC?
The U.S. CDC currently produces CVVs according to WHO guidelines, with support from Quality Management Systems (QMS) and Quality Assurance Unit (QAU) within the CDC Influenza Division to ensure compliance with applicable regulations.
The production process begins with ensuring a decontaminated workspace that is free from viruses, bacteria, and any other potential contaminants. Prior to the start of any experimental procedure, the QAU rigorously inspects all raw materials and reagents while maintaining detailed documentation that is subsequently reviewed by another subject matter expert for completeness, accuracy, and compliance.
Through global surveillance efforts coordinated by the WHO Global Influenza Surveillance and Response System (GISRS), scientists identify influenza viruses with pandemic potential, including novel hemagglutinin (HA) and neuraminidase (NA) gene sequences, both of which are surface influenza glycoproteins. These sequences are then cloned into influenza reverse-genetics plasmids, which are purified and sequenced to confirm the presence of the HA and NA sequences of interest.
Once the appropriate plasmid has been selected, Vero cells are transfected through electroporation or a lipid-mediated transfection using lipofectamine to initiate virus production. Due to the relatively low virus-rescue efficiency of Vero cells, isolated and validated viruses are introduced into embryonated chicken eggs, enabling mass production and downstream manufacturing.
To confirm functional virus production, the HA assay is performed and eggs with the most diluted HA content are selected to generate a working stock. High-dilution eggs minimize the potential of proliferation of mixed viral populations and/or defective interfering particles.
To ensure that a CVV meets the required quality criteria for downstream use, each CVV is evaluated for key attributes including identity, purity, titer, genetic stability, antigenicity, attenuation, and, where applicable, biosafety characteristics.
Conclusions
This protocol provides a transparent guide on how the U.S. CDC produces influenza CVVs for their potential deployment during a pandemic emergency. To date, this protocol has supported the production of more than 100 CVVs, 36 of which have been distributed to vaccine manufacturers, academic researchers, and other stakeholders.
Although this method is considered a reliable and robust approach for large-scale CVV production, it is associated with notable limitations, including the emergence of egg-adaptive mutations, supply vulnerabilities, and variable production quantities. In response to these challenges, recent advances in the development of novel vaccine platforms, including recombinant protein- and nucleic acid-based systems, provide alternative manufacturing strategies that enable rapid antigen redesign to ultimately improve pandemic responsiveness.
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