Chicken Eggs are Vital for Scientists at the Center for Disease Control

Why the Flu Never Flies

Chicken Eggs are Vital for Scientists at the Center for Disease Control

By Heather & Forrest Townsend

Most of us do not put much thought into where a vaccine comes from. Sure, it is scientists in white coats, gloves and goggles, tirelessly working to hopefully save millions of people’s lives. But have you ever wondered what role your beloved chickens might play in the development if those vaccines? You might want to hug your chickens a little harder than usual tonight after you read this.

Chickens have an amazing role in the development of vaccines. Have you ever been asked, “Before you get this flu vaccine, are you allergic to eggs?” It’s because of the remarkable incubation chamber that the chicken egg provides that has led to the mass production of vaccines on the market today. We are not here writing this article as pro-vaccine or anti-vaccine scientists, but merely relaying the facts of how chickens are used in this process. Years of teaching microbiology (Heather) and taking care of peoples’ beloved pets (Forrest) have allowed us to appreciate all that goes into protecting one’s immunity.

Why Eggs?
Viruses are very small nonliving particles. They are not cells; other microorganisms such as bacteria, fungi and protozoa are cellular organisms and contain all of the features necessary to carry out life. Viruses do not possess all of those necessary characteristics. For instance, if a scientist wanted to study antibiotic-resistant bacteria, all they would need would be a petri dish with some nutrients (called agar), and they could grow the bacteria on a lab bench. Then, when it came time to test its sensitivity to an antibiotic, they would simply add the antibiotic and see the effect that it had on the bacteria. Viruses are much trickier to grow and to use to test various substances.

Viruses require a host cell in order to reproduce. If scientists wanted to grow viruses in order to produce a new vaccine, they need living cells for the virus to get into, gather all of the necessary materials to make more of itself, and then leave the cell to take over more. For this reason, it is impossible to culture viruses on nonliving media, such as a petri dish. Often times in laboratory settings, they will use various cells growing in a petri dish and offer them to the viruses so that they may infect them and reproduce (the more viruses one has to study in a lab, the better). A chicken egg is a perfect place for this to happen. The embryonated egg acts as a controlled environment with all of the cells that a virus would need in order to successfully replicate.

There are many various types of viruses. The influenza virus is an RNA virus that has many different strains. Influenza causes approximately 200,000 Americans to be hospitalized each year and is the cause of death of approximately 36,000. Historically, the pandemic of 1918 was the worst and most deadly — 500 million (a third of the world population at that time) people contracted the disease and an estimated 20 to 50 million died. This pandemic of 1918 occurred prior to the advent of the influenza vaccine, which was developed in 1933. Today, the most common way the influenza vaccine is produced is through chicken eggs.

An Annual Need
So why do you have to get a new flu shot every year? Since the influenza virus mutates frequently and new strains arise, it is crucial to develop a new vaccine for use each year.

The vaccine helps protect people from contracting the virus and can decrease the severity of the symptoms associated with flu. Influenza can cause symptoms such as coughing, fever, fatigue, muscle aches, sore throat, and even vomiting and diarrhea.

Most vaccines like the flu vaccine contain a particle that resembles a disease-causing microorganism, a killed microorganism, or a live weakened virus. These particles cause our immune system to react and protect us from these organisms when we are exposed to the real virus. Since our body responds to specific parts on the influenza particle, it is important to make a new vaccine each year to ensure that we are protected due to the ever-changing influenza virus. (This is one reason why when the H1N1 pandemic outbreak happened in 2009, we were not equipped to handle it in a quick manner — it took time for us to develop a vaccine for this relatively new strain of the virus.)

Currently, there are three approved ways the U.S. Food and Drug Administration (FDA) allows the production of the influenza vaccine. There is the recombinant flu vaccine, the cell-based flu vaccine, and the egg-based flu vaccine.

Using chicken eggs in the production of a flu vaccine is the most common way that the influenza virus is made. The process of harvesting the influenza virus inside a chicken egg has been around for more than 70 years and is considered safe. Using eggs to produce a vaccine results in a flu shot (which consists of using inactivated viral particles) and a nasal spray (which uses a live but weakened virus).

The Vaccination Process
The Centers for Disease Control (CDC) is the governmental office in charge of all things infectious and disease-related. Based in Atlanta, Georgia, this is the agency that keeps tracks of outbreaks and all sorts of data.

The CDC starts the process of producing the egg-based influenza vaccine by selecting three strains of the virus that are projected to be circulated throughout the population in the upcoming flu season (typically the winter months). The CDC distributes these strains to private manufacturers.

Private manufacturers obtain fertilized chicken eggs and incubate these eggs for 10 to 11 days at 98.6°F to allow for cell growth to occur. The viruses are injected into a small hole that is made in a non-veined area of a fertilized chicken eggs. Different eggs are used for the three different strains since they want each egg to produce a pure strain of the virus. The hole is then covered using parafilm (a piece of wax-like paper) and the virus is left to reproduce for two to three days. For the flu shot preparation, a chemical is injected into the egg to kill the virus and then the allantoic fluid that contains the virus is extracted. For the nasal spray, the viral particles inside the egg are weakened (the process for making this is different than the flu shot, and we will focus on the flu shot preparation). The FDA tests for purity of the viral particles, and it then eventually combines the three strains and creates the vaccine. The production of the vaccine is an entirely different process that requires a lot of checks and balances along the way.

Influenza vaccines produced in this way are considered safe and effective at supplying someone with the immunity they need to protect against the influenza virus. Since the vaccines will contain a small amount of egg protein, it is recommended that patients with severe reactions to eggs not receive the vaccine until they have gone through appropriate sensitivity tests. (So there is the answer to the doctor’s question about your sensitivity to eggs and egg products.) Other vaccines, such as Measles-Mumps-Rubella (MMR) and herpes simplex virus have also been produced using the egg-based method. Estimates have shown that it would take approximately1.2 billion chicken eggs to produce three billion doses of a vaccine!

So there you have it. Our chickens protect us from the inevitable boringness of life, and also help to protect against a potentially deadly disease.

Heather is a professor at a community college, where she uses her PhD in veterinary medical sciences to teach microbiology (and in the past, zoology). Forrest is a veterinarian. Both are avid chicken enthusiasts who said they breed Silkies because, “We have found they are the best with our three children.”

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