Eggs in vaccines? Did you know that many vaccines contain eggs?

It may sound strange, but it is true- many popular vaccines are manufactured using eggs. This vaccine manufacturing method has been used for decades, ensuring that millions of people around the world have access to safe vaccines.

Some common flu vaccines contain trace amounts of egg because the vaccines are either cultured in eggs or chick embryos.  Vaccines that are produced via these methods often contain small amounts of ovalbumin. Many common vaccines, including the vaccines for rabies, influenza (the flu) and yellow fever are manufactured using egg-based methods. Manufacturing the influenza vaccine through egg-based methods is the oldest way of making flu vaccines.

Why use eggs for vaccines?

In the USA, egg-based vaccines are the most common method used for making flu vaccines and this manufacturing process has been used for more than 70 years. Initial research into the use of eggs for vaccines originated in the 1930s, with the first trials taking place in England in 1937.  Egg-based manufacturing methods are used to produce both the inactive vaccine, as well as the live attenuated vaccine.

For many years, egg-based vaccine production has been the only way to supply enough volume of vaccines to meet global health needs. There are critics of this vaccine who oppose the method because avian cells display different proteins compared to human cells, which could affect how the virus mutates.

In order to make vaccines through eggs, scientists inject a live virus into an embryonated egg, then allow the virus to incubate and replicate, much like the virus would do in a human host. Once replicated, the scientists collect the replicates, purify them, and then kill them. The antigens are then used to make the flu vaccine. Once produced, the vaccine needs to undergo a rigorous screening and approval process. The US Centre for Disease Control and Prevention (CDC) and other international labs partner with the World Health Organization (WHO) to choose certain virus strains that they send to private vaccine manufacturers.  This process takes place annually because the flu can mutate, and strains can change frequently. This means that scientists need to manufacture and distribute new vaccines every flu season.

Can egg-based methods be used to develop coronavirus vaccines?

As labs around the world have scrambled to develop an effective coronavirus vaccine, questions were raised regarding whether egg-based production methods would be relevant for the current Covid-19 pandemic.  Experts have advised that egg-based methods would not be suitable for the production of a coronavirus vaccine, because the coronavirus has different receptors and other qualities and cannot be replicated inside eggs the way that some other viruses such as flu can.  Other methods have been used to formulate the coronavirus vaccines currently in use around the world.

Are Vaccines made with eggs safe for individuals with egg allergies?

Different egg-based vaccines contain varying levels of egg protein. Some studies have found that egg-containing vaccines can pose potential risks to individuals with sensitivities to eggs, particularly young children, though the chances of a severe allergic reaction are unlikely. Individuals should consult their personal physicians to make informed medical decisions based on their medical history and consult the guidance of experienced professionals to make informed choices.

Egg-free alternatives are available for certain egg-based proteins, for individuals who are worried about the potential effects of the egg protein in egg-based vaccines.