This is Public Health

The flu shot

With fall and winter also comes flu season in Canada. Information on avoiding the flu and preventing the spread of germs is everywhere. One of the most talked-about parts of flu season is the flu shot—and it comes with lots of questions. Should I get one? Will it make me sick? Why do I have to get one every year? Here are some of the most frequently-asked questions about the flu shot, answered.

Will the flu give shot give me the flu?

No, there are two types of flu vaccine, an injectable flu shot and a nasal spray flu vaccine, and neither can give you the flu.

The vaccine found in flu shots contains deactivated viruses and cannot cause the flu. The most common side effects of flu shots are soreness, redness, tenderness or swelling where the shot was given (Nichol et al., 1996). 

The nasal spray flu vaccine contains weakened viruses that cannot cause the flu. The viruses are also cold-adapted—meaning they can only survive in the cooler nasal areas. They cannot survive in deeper areas of the body where warmer temperatures exist (CDC, 2015). The most commonly reported side effects of the nasal spray are a runny nose, congestion and cough.

Why do some people feel flu-like symptoms after receiving a flu shot?

Less than 1 per cent of people who receive the flu shot will develop low-grade fever, headache and muscle aches (Immunization Action Coalition, nd). However, this is not because they have the flu. 

There are several reasons why you might get—or feel like you are getting—the flu, even after being vaccinated.

  1. You may have a respiratory infection, like a cold.
    Colds cause symptoms similar to flu, and tend to spread between November and March. The flu vaccine only protects against flu viruses, not cold viruses.
  2. You may have contracted the flu virus shortly before or after receiving a vaccine.
    Your body takes one to two weeks to develop immunity to the flu after receiving the flu shot. So you may still get sick if you come into contact with the flu virus before the protection from the vaccine is in place.
  3. You may have come into contact with a flu virus that is different from the viruses in the vaccine.
    Unfortunately, the seasonal flu vaccine will not protect you if the virus you contract does not match one of the strains in the vaccine. Flu viruses travel and change each year. Scientists track the movement of viruses and how they change to best estimate which viruses will reach our area come flu season. Vaccines are developed to protect from multiple strains of flu viruses.
  4. You may have a weakened immune system.
    Unfortunately, flu shots do not always give sufficient protection for those who have weakened immune systems or people age 65 and older. This is why it is important for everyone to receive vaccinations to prevent the spread of disease and infection to vulnerable people. (CDC, 2015).

Why does the flu come in seasons around the same time of year? Does it come at different times in different places around the world? 

Various factors combined result in the new flu outbreaks each year:

  • The Environment
    • Flu viruses like cool and dry weather. This is why flu season in Canada takes place in autumn and winter. They survive longer in dry air than in wet. Also, they hold out better when left to wait exposed on cool surfaces—like doorknobs and keyboards (Tamerius et al., 2013).
  • Virus Behaviour
    • Flu viruses travel and mutate as they come into contact with one another. The mutations prevent our immune systems from recognizing flu viruses from one year to the next (Nelson et al., 2007).
  • Human Behaviour
    • Our winter festivities and indoor behaviours also create easy spread of flu viruses. Christmas parties, indoor sporting events, like hockey games or attending classes in school or at university, increase our contact with others and the flu virus (Lofgren et al., 2007).
  • Sunshine
    • Short daylight hours and indoor activities weaken our immune system, leaving us more susceptible to illnesses such as the seasonal flu infection Dowell, 2001).

Do I really need to get the shot every year?

Yes. The flu virus changes each year, so the flu shot you receive this year will not protect you from next year’s flu. Protection from the flu shot declines over time so getting a shot every year will give you the best protection against the flu. Health Canada recommends a yearly flu vaccine for everyone 6 months and older (Health Canada, 2015).

Is it really that important that I and the people I live with get the flu shot? 

In short, yes. 

First, the flu is contagious whether or not you feel sick. You can infect other people beginning one day before symptoms develop and up to a week after becoming sick (Carrat et al, 2008). Children are more contagious. They may be able to infect others for over a week (Frand et al 1981). People with especially strong immune systems may never show their symptoms, but are still able to pass the illness to others.

Second, living in a household makes for the spread of the flu virus very easy. Flu can move between two people up to six feet apart. The flu is most easily passed on by a person talking, coughing or sneezing. People can also get the illness by touching a surface or object that has flu virus on it and then touching their mouth or nose (CDC, 2015).

Third, if you are living with people, you may directly or indirectly infect someone who could have severe reactions to the flu virus. Pregnant women, small children, people older than 65 or those with a weakened immune system are far more likely to be hospitalized if they contract the flu (Simonsen et al 2000).

Is it better / safer to just get the flu naturally?

Flu can be a serious disease, particularly among young children, older adults, pregnant women and people with weakened immune system (Simonsen et al. 2000).

Risks of any flu infection include serious complications, hospitalization or death, even in normally strong and healthy children and adults. Even if fighting the flu infection does not cause severe symptoms for you, you may infect others causing serious health complications for them.