Dear VaccineSmarts,

I got my first flu shot a few years ago. I felt sick right after it but that has never happened again. Why did that happen? How does the vaccine work? Is it like a vitamin that boosts the immune system?



Dear Frank,

Those are good questions. Most people who aren’t in a medical or science profession were never taught about the immune system or how vaccines work.

Our immune system is always on the lookout for invaders such as viruses and bacteria. The first time the immune system detects an intruder, it reacts in rather nonspecific ways.

The immune system releases chemicals that activate different kinds of immune cells. Oftentimes, the chemicals that are released make people feel ill with headaches, muscle pains, tiredness and fevers.

The immune cells try to contain the invader and keep it from multiplying and spreading. When it fails to do so, we develop the symptoms associated with the foreign intruder.

If the invader causes vomiting and diarrhea, we get vomiting and diarrhea. In the case of the flu, we also may develop symptoms such as cough, sore throat and runny nose.

The immune system doesn’t give in. It continues the battle by making antibodies. Antibodies are special proteins that attach to particular parts of an invader.

Antibodies help the immune system destroy the invader and they are very specific. For example, there are a set of specific antibodies for each strain of the flu. Antibodies for one strain of flu usually won’t attach or attach very poorly to another strain.

The immune system sets up memory cells to remember the invader after the illness is over and the invader is removed. The memory cells hang around and keep making antibodies. If the invader shows up again, the antibodies attach and the invader is destroyed before it can make you sick.

The flu vaccine takes advantage of the immune memory cells. The injectable vaccine is made from only one part of the flu virus. It is a part that helps the virus to enter our cells.

Once injected, that part of the virus is recognized as an invader. The immune system releases the chemicals that activate the different immune cells. Antibodies are produced against the virus part in the vaccine.

After about two or three weeks, there are enough antibodies and memory cells to protect a person from the strains of flu covered by the vaccine.

As mentioned earlier, the chemicals released in the initial immune response to the vaccine may make a person feel ill. If symptoms occur, they should only last a day or so because the vaccine only contains a tiny amount of the material from the virus.

Some believe the vaccine gives them the flu, which is not possible because the whole flu virus is not in the vaccine.

It also is not possible that the injectable vaccine would cause a cough, sore throat and runny nose that are associated with the flu.

It is important for people who don’t feel well from vaccination to remember that the symptoms are short-lived and minor compared to a full-blown case of the flu.

The most important thing to remember is that the flu vaccine works by stimulating our immune system to produce antibodies.

The antibodies are specific to the flu strains in the vaccine. The flu shot is different from year to year because the flu strains that are around change. Therefore, it is important to get vaccinated each season.

Dr. Richard Rupp is a pediatrician and member of UTMB’s Sealy Center for Vaccine Development. Bridget Hawkins, Ph.D. is the assistant director of the SCVD. This column is supported by a UTMB President’s Cabinet Award to provide information about vaccines. Visit or like us on Facebook.

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