Influenza viruses are always changing, which means that the flu vaccine is updated every year. The Centers for Disease Control and Prevention (CDC) recommend that everyone over 6 months of age (with some exceptions) should have a flu vaccine each year.
Different forms of flu vaccine can have slightly different ingredients. For instance:
- An injection usually contains tiny amounts of deactivated (therefore not harmful) flu viruses.
- A nasal spray contains live viruses that have been weakened and are therefore not harmful. The CDC did not recommend the use of nasal spray for the 2017–2018 flu season.
What is in a flu shot and why?
Many vaccines contain similar ingredients, including a small amount of the virus or bacteria being vaccinated against.
Many vaccines for flu and other viruses contain similar ingredients. The purpose of each ingredient is to either make the vaccine effective or ensure that it is safe.
Many studies over the years have shown that flu vaccines are safe. The CDC say that it is the best way to avoid getting the flu and spreading it to other people.
The following list looks at seven ingredients in vaccines, including flu shots, and why vaccines need them:
1. Influenza viruses
The flu vaccine contains tiny amounts of the viruses it protects against. The presence of these viruses in the vaccine triggers the body's natural defense mechanism to produce antibodies to fight them. This means that the body quickly recognizes them when exposed to the disease in "real life."
Different influenza viruses contained in a flu shot are:
- influenza A virus H1N1 called the Michigan strain
- Influenza A virus H3N2 called the Hong Kong strain
- one or two influenza B viruses called the Brisbane and Phuket strains
Traditional flu shots are trivalent (three-component) vaccines because they protect against three viruses: two influenza A viruses (H1N1 and H3N2), and one influenza B virus.
A person can also get a quadrivalent (four-component) vaccine, which protects against an additional B virus.
Formaldehyde is toxic and potentially lethal in high doses. However, it is present in such small amounts in a flu vaccination that it is harmless.
Formaldehyde's role in the flu shot is to inactivate toxins from viruses and bacteria that may contaminate the vaccine during production, as well as the viruses naturally present in the vaccine.
Formaldehyde is typically present in the human body and is a product of healthy digestive function.
3. Aluminum Salts
Aluminum has been used in vaccines for over 70 years.
Aluminum salts are adjuvants, meaning that they help the body to develop a stronger immune response against the virus in the vaccine. Because they boost the body's response, this means that the vaccine can contain smaller amounts of the virus.
Similar to formaldehyde, and to most ingredients in the flu shot, the amount of aluminum present in the vaccine is extremely small.
This compound is not always present in flu vaccines, some of which are aluminum-free.
Thimerosal is not found in all flu vaccines.
Thimerosal is a preservative that keeps the vaccine free from contamination by bacteria and fungi. Without this, the growth of bacteria and fungi is common when a syringe is in a multi-dose vial (a vial that contains more than one dose).
Thimerosal is made of an organic form of mercury known as ethylmercury, a safe compound that usually only stays in the blood for a few days.
It is different from the standard mercury that can cause illness in large doses, and from the mercury found in seafood (called methylmercury), which can stay in the body for years.
Flu shots will only contain thimerosal when they are in a multi-dose vial. Single-dose vials, pre-filled syringes, and nasal sprays do not need to include this preservative because contamination is not an issue.
5. Chicken egg proteins
Proteins from chicken eggs help viruses to grow before they go into the vaccine.
Influenza viruses used in vaccines are usually grown inside fertilized chicken eggs, where the virus makes copies of itself. After that, the viruses are separated from the egg and placed in the vaccine; this means that the finished vaccine may contain small amounts of egg proteins.
The CDC say that the flu shot is typically safe for people with egg allergies, but those who have an egg allergy must mention it to the doctor before receiving the shot. A person with a severe egg allergy may require monitoring by a doctor following the injection.
Egg-free flu shots are also available.
Gelatin is present in the flu shot as a stabilizer. Stabilizers keep the vaccine effective from the point of production to the moment of use.
Stabilizers also help to protect the vaccine from the damaging effects of heat or freeze-drying.
Most flu vaccines use pork-based gelatin as a stabilizer.
Antibiotics are present in the flu vaccine to keep bacteria from growing during the production and storage of the vaccine.
Vaccines do not contain antibiotics that may cause severe reactions, such as penicillin. Instead, vaccines contain other forms of antibiotics, such as gentamicin or neomycin. Neomycin is also an ingredient in many topical medications, such as lotions, ointments, and eye drops.
Benefits of flu vaccine
Having the flu vaccine has several benefits, including:
- Preventing a person and those around them from getting sick with flu.
- Reducing the risk of hospitalizations from flu, particularly among children and older adults.
- Protecting vulnerable groups of people, including babies and infants, older people, and people with chronic diseases.
- Protecting women during and after pregnancy, by reducing the risk of flu-associated acute respiratory infection, and reducing the risk of the baby getting the flu.
- Preventing complications in people who have chronic diseases. The vaccine decreases the rate of major heart problems in people with heart disease and the rate of hospitalizations in people with chronic lung disease and diabetes.
Who should avoid flu shots?
Those who are feeling unwell should delay getting a flu shot until they feel healthy again.
The CDC recommends that everyone over the age of 6 months should receive the flu vaccine every year, though they also provide guidelines of who should avoid the vaccine or take precautions.
Age, current and past health, and allergies to the flu vaccine or its components are factors to consider.
The following people should avoid or be cautious of the flu vaccine:
- children under 6 months of age
- people with severe allergies to any of the flu shot ingredients, such as gelatin, antibiotics, or eggs
- people who experienced a severe allergic reaction from a previous flu shot
- people who have had Guillain-Barré Syndrome
- people who are not feeling completely healthy
Side effects of flu shots
The flu vaccine cannot cause flu because it is made with inactivated ("dead") or weakened viruses, which are not infectious, or with synthetic, lab-made variants of the flu virus.
However, a flu shot may cause slight flu-like symptoms that can last for a few days, such as:
However, the most common side effect is a slight soreness or redness in the arm at the site of the injection.
In rare situations, the flu vaccine can cause serious side effects, such as allergic reactions. These usually occur within a few minutes to a few hours after vaccination and can be treated.