Many of us have experienced symptoms such as a sore arm or a fever after receiving vaccinations. Feeling a little under the weather can make some people hesitant to get a jab in the future. But these aftereffects are actually a good thing and an important part of how our bodies develop immunity to the virus the vaccine is protecting you from. The symptoms we feel after getting a shot is called reactogenicity. They happen because your immune system is responding to the vaccine.

Why vaccines can make you feel bad

The goal of vaccines is to activate the immune system. Immune cells throughout the body have special recognition receptors that respond to molecules in vaccines and initiate the immune response within minutes of injection. Cells in the body release substances that cause inflammation and widening of the blood vessels around the injection site, which increases blood flow to the area. This can lead to soreness and redness on your arm, but it also directs more immune cells to the injection site.

The inflammatory markers that are released around the injection site also travel through the bloodstream and signal to the brain that the body is under attack. One way your body reacts to the perceived attack is to “raise the thermostat,” which means you may run a fever.

During the post-vaccine immune response, specialized cells read and remember these harmless molecules in the vaccine so if or when the real virus strikes, they know exactly what it is and how to eliminate it quickly and effectively before it causes any damage. The more activated your immune system is by a vaccine, the more cells will recognize the virus and the more protected you will be later on.

What can you do to feel better?

If you’re not feeling great after getting a vaccine, here are some things you can do:

• Apply ice to the injection site if it’s sore.
• Avoid alcohol and tobacco, as they can worsen side effects.
• Take ibuprofen or other over-the-counter pain relievers after you begin to feel symptoms. But don’t take them before your vaccine—it may lessen your immune response.

Vaccines ultimately work by tricking your body into overreacting to a harmless substance so it’s prepared for what it may have to face in the future. While you may feel lousy for a day, remember your immune system is working hard, which will keep you—and others around you—protected from a much more dangerous infection down the road.