How Flu Shots Work and Why They Sometimes Don't

Everyone over the age of 6 months should get a flu shot each year in order to protected against the strains predicted to be circulating through the flu season. As this is different from many immunizations where you get a shot once or twice in a lifetime, it is important to understand why this is needed for the influenza vaccine.

Your Immune System and the Flu

Your white blood cells respond to substances they consider to be foreign (antigens) and produce antibodies, which are special proteins that can latch onto these substances. This can prevent a virus from being able to enter and infect or cell, or may alert other white cells to come and attack the invader.

This happens naturally when you are infected by the influenza virus, although it takes a couple of weeks and can't stop you from getting sick with the virus the first time you are exposed. The goal of the flu vaccine is to expose your immune system to the viral antigens before you are infected. In this way, your body will have antibodies ready to go when you are exposed to the flu in your community. As a result, you may not get sick at all or you may have a milder case of the flu.

How the Flu Virus Changes

There are different types of flu viruses, including influenza A and influenza B, and each has subtypes. Influenza viruses mutate constantly, and this is one reason that yearly flu shots are necessary. The mutations change the surface proteins that are the antigens targeted by your antibodies. As a result, even though you made antibodies against a previous strain, these won't work to deactivate the new strain.

The antigens of the influenza virus are hemagglutinin (HA) and neuraminidase (NA). They are used in naming conventions for influenza, such as H1N1. The flu shot targets the HA antigen, while the nasal spray flu vaccine targets both.

The virus can change in two different ways. A slight change is called a “drift,” while a major change it is called a “shift.” Interestingly, only influenza A viruses can mutate by shifts. This is seen when a human flu virus crosses with a flu virus that usually infects animals such as pigs or birds. When there is a shift, very few people have any immunity to the new strain and it has the potential to become a pandemic.

Making the Annual Flu Vaccine

The flu vaccine takes at least six months to make, so it is no small feat to get it prepared for the beginning of flu season each year. The formula for the vaccine is developed during the previous year’s flu season.

Researchers do ongoing surveillance to look at the strains of influenza that are circulating and how they have been mutating. They try to determine the strains that are most likely to be causing illness during the following flu season. Three or four different strains of the influenza virus are chosen to be included in the vaccine to (hopefully) provide as much protection to as many people as possible.

Once the strains are chosen, manufacturers begin developing the vaccine. In fact, some manufacturers may begin to do this up to a month before the new formula is announced so they will be prepared and have enough time to make adequate amounts.

The traditional way the vaccine is manufactured is by inoculating fertilized chicken eggs with the chosen viral strains. After a few days, fluid from the eggs is harvested and purified to make the vaccine. Both inactivated influenza vaccine and live attenuated influenza vaccine (the nasal spray flu vaccine) are made in this way. It is challenging to maintain flocks of poultry to provide the eggs, plus it takes time to develop the candidate strains that grow well in the eggs. Some influenza viruses, such as the H3N2 viruses, grow poorly in chicken eggs.

A process was approved in 2012 to allow growing the virus on cell cultures. This results in an egg-free vaccine that is safer for people who are severely allergic to eggs.

A recombinant process was approved in 2013. It has the promise of quicker production of vaccine than the egg-based and cell-based processes. It uses only the viral DNA that codes for the HA antigen production, adding them to a baculovirus which then is grown on cell line, producing the antigens in bulk. As of the 2019-2020 flu season, only one influenza vaccine is produced this way in the U.S.

Advances in Flu Vaccine

The cell-based and recombinant processes for manufacturing the vaccines are two significant advances. There are also developments in making vaccines for special populations and circumstances.

For those age 65 and over, high dose vaccines and adjuvant vaccines have been developed. As the immune system's response wanes with age, these vaccines are designed to produce a stronger response. They have also developed an intradermal vaccine that uses less antigen, therefore stretching the vaccine supply when necessary. It is injected under the skin rather than into the muscle.

Research is ongoing to find a universal flu vaccine that will be effective against all strains. This could eliminate the need to develop a new vaccine each year, or even allow people to get the vaccination less often than yearly.

Why the Flu Shot Doesn’t Always Work

Typically each year, one or two of the viruses in the flu shot are updated to anticipate mutations. However, if a major shift occurs, or the virus mutates to a different form than what researchers anticipated, the shot may not cover some of the circulating viruses. If there are viruses circulating that are not covered by the vaccine, you may still get the flu even if you had a flu shot.

The good news is that the antibodies your body produces to fight the viruses in the flu shot are typically able to provide some resistance to mutated versions of that virus. Even if it does not prevent the illness, you are more likely to have a less severe case of the illness if you have had a flu shot.

What You Can Do to Prevent the Flu

If you haven’t been vaccinated against the flu, there are still steps that you can take to protect yourself. The simplest is to get the flu shot. Although it is typically recommended that people get flu shots in the fall, they can still be effective if you get them in the winter or spring, especially if flu is circulating in your area. It takes about two weeks to get protection from a flu shot.

Hygiene measures can also stop the spread of flu. Frequently wash your hands with soap and water. If those aren't available, use an alcohol-based hand sanitizer. Cover your cough to avoid transmitting your sickness to others. Limit your contact with others and stay home if you are sick (at least until your fever has been gone for 24 hours without using fever-reducing medicine).

For many people, taking antiviral medication such as Tamiflu (oseltamivir) or Relenza (zanamivir) within the first 48 hours of flu symptoms will help reduce the severity and length of the illness. Because these drugs are only available by prescription, you will have to see your healthcare provider. If you have been exposed to someone with the flu, an antiviral medication may be prescribed to help protect you from getting the virus.

A Word From Verywell

Take every step you can to protect yourself, your family, and your community from the flu. It's not a mild or minor disease. For example, in the 2017-2018 flu season, 45 million Americans were sickened and 61,000 died of influenza. You have the ability to reduce the risk.

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  1. Centers for Disease Control and Prevention. Key facts about seasonal flu vaccine. Updated December 2, 2019.

  2. Centers for Disease Control and Prevention. Preventive steps. Updated October 9, 2019.

  3. Centers for Disease Control and Prevention. How the flu virus can change: “drift” and “shift.” Updated October 15, 2019.

  4. Centers for Disease Control and Prevention. Selecting viruses for the seasonal influenza vaccine. Updated September 4, 2018.

  5. Centers for Disease Control and Prevention. How influenza (flu) vaccines are made. Updated December 12, 2019.

  6. Centers for Disease Control and Prevention. Influenza vaccine advances. Updated September 16, 2019.

  7. University of Chicago Medicine. Why doesn't the flu vaccine work sometimes? Updated February 20, 2018.

  8. Cleveland Clinic. 7 answers on Tamiflu: is it best to help you fight flu? Updated January 8, 2020.

  9. Centers for Disease Control and Prevention. 2017-2018 estimated influenza illnesses, medical visits, hospitalizations, and deaths and estimated influenza illnesses, medical visits, hospitalizations, and deaths averted by vaccination in the United States. Updated November 22, 2019.