The influenza vaccine protects individuals from the seasonal flu virus and its potential complications. Each year, the vaccine is updated to target specific strains, helping the immune system develop defenses. Manufacturing these vaccines relies on various biological systems, called “hosts,” used to cultivate the influenza virus or its components. The selection and use of these hosts are important for producing safe and effective vaccines.
Traditional Egg-Based Production
For over 70 years, fertilized chicken eggs have been the primary method for producing influenza vaccines. This established technique begins with injecting a candidate vaccine virus into fertilized chicken eggs. The eggs are then incubated for several days, allowing the virus to replicate within the embryonic fluid.
After incubation, the virus-containing fluid, known as allantoic fluid, is harvested. The viruses are then purified and inactivated, meaning they are killed so they cannot cause illness but can still trigger an immune response. This method accounts for a significant portion of the U.S. flu vaccine supply. However, challenges include the process being time-consuming and the potential for the virus to undergo minor changes as it adapts to grow in eggs, which can sometimes affect vaccine effectiveness.
Modern Cell-Based Production
Modern cell-based production offers an alternative to egg-based manufacturing. This method grows influenza viruses in cultured mammalian cells instead of eggs. The process involves inoculating these cell cultures with vaccine viruses, allowing them to replicate for a few days.
Once the viruses have multiplied, the fluid containing them is collected, and the viral antigen is purified and inactivated. Cell-based production offers several advantages, such as faster start-up times for manufacturing, which can be beneficial during a pandemic. It also eliminates dependence on egg supply fluctuations and may result in vaccine viruses that are a closer genetic match to circulating wild-type strains, as there is no egg adaptation process.
Recombinant Technology for Vaccines
Recombinant technology is another approach to flu vaccine manufacturing. This method does not require growing the entire influenza virus. Instead, it isolates the genetic material for a flu protein called hemagglutinin (HA). HA is a surface protein on the influenza virus that triggers the human immune system to produce protective antibodies.
The HA gene is combined with a baculovirus, a type of virus that infects insects but not humans. This recombinant baculovirus then infects insect cells. These insect cells act as biological factories, producing large quantities of the HA protein. The purified HA protein is then formulated into the vaccine, offering an egg-free option that can be produced rapidly and with high precision.
Advancements in Vaccine Manufacturing
The development of diverse host systems for flu vaccine production improves manufacturing capabilities and addresses public health needs. These technologies, including traditional egg-based, cell-based, and recombinant methods, are used concurrently to enhance vaccine availability and effectiveness. One main reason for this diversification is the need for faster production timelines, especially in response to potential pandemics, where rapid vaccine deployment is important.
Another factor is the aim to mitigate issues like egg allergies, as some newer methods are entirely egg-free. The evolution of these host systems also helps improve the genetic match between the vaccine strains and circulating wild-type viruses, potentially leading to enhanced vaccine effectiveness. Research continues to explore and refine these and other emerging technologies to optimize flu vaccine production, ensuring a more robust and adaptable global supply chain.