Some eggs begin and end their lives on the breakfast plate, scrambled, boiled, or fried, but others are destined for a greater purpose. Your standard, white chicken egg is an essential ingredient in flu vaccines. In fact, it can take up to 360,000 eggs a day to create the vaccines we rely on to ward off the influenza virus this time of year. Pharmaceutical company GSK opened their doors to Mosaic to reveal how the humble egg factors into the flu vaccine manufacturing process. These eggs, which come to this branch of GSK from farms throughout Germany and the Netherlands, wind up doing arguably more for your health than the eggs you eat.

You may be familiar with the basic mechanizations of a vaccine: A version of a virus or another organism is entered into the body to stimulate immunity against that same organism when encountered in the real world. In the case of the flu vaccine, the eggs are key to the process that transforms a harmful influenza virus into the substance that stimulates the immune system, known as the antigen. In a video for Mosaic, a scientist from GSK explains the process step by step.

First, the team at GSK implants the influenza virus into an egg. Then, they incubate the egg under controlled conditions. While the egg incubates, the influenza multiplies. When the incubation phase is complete, the team harvests fluid that contains millions of copies of the virus from the eggs. Once the fluid is harvested, the virus is separated from the fluid, split, and inactivated to prevent any further replication. The last step is the most complicated. As told to Mosaic, the scientists “sterile filter to yield a sterile suspension of purified influenza split virus with each of the four influenza strains we receive from the WHO reference laboratories, in order to formulate a quadrivalent vaccine containing two B strains and two different A strains as a base composition for this vaccine.”

The whole process from start to finish takes 30 days, followed by another 30 days to complete the vaccine formulation, filling, and packaging. The final flu vaccine will sometimes contain small amounts of egg protein, and although flu vaccines that contain egg proteins are unlikely to trigger an allergic reaction, egg-free vaccines are available. It’s always best to mention any egg allergies to your doctor before being vaccinated. 

This year, when bracing for a flu vaccine shot, you can thank the hundreds of thousands of eggs that helped make it happen.