What the Flu Vaccine Does
The goal of an influenza vaccine is to expose your immune system to viral proteins. This may sound like exactly what you are hoping to avoid, but these proteins serve as antigens—substances your immune system will form defensive antibodies to combat.
When you are next exposed to an influenza virus (say, an infected person sneezes near you), your body will be ready to make antibodies that will prevent you from being infected. The antibodies latch onto the virus and inactivate it.
Types of Flu Vaccines
There are a few different flu vaccination options. While formulated differently, they all work to achieve this result. And despite being derived from the influenza virus itself, they do this without making you sick:
The injected flu vaccine (the flu shot) is made from the influenza virus grown in a culture medium (eggs or a cell line). The virus is then killed and purified before it is made into the vaccine you receive. This is an inactivated vaccine, meaning no infective virus remains. There is also a recombinant flu vaccine that isolates the gene that makes viral protein antigen and allows the manufacturer to make just that protein for the vaccine rather than the virus itself. This is purified, and there is no risk of being infected with the influenza virus. The nasal spray flu vaccine is made with a live but weakened influenza virus (called a live attenuated virus). While it contains a live virus, the U. S. Food and Drug Administration (FDA) ensures it is safe for healthy, non-pregnant people 2 through 49 years of age.
Why You May Feel Sick After a Flu Shot
While the flu vaccine cannot give you the flu, it can have mild side effects that mimic influenza. This stems from your body producing the desired immune response.
These side effects usually occur soon after the vaccine is administered and last one to two days. Both the injected flu vaccine and the nasal flu vaccine may produce the following symptoms:
Soreness, redness, or swelling at the injection site Low-grade fever Muscle aches Headache Nausea Fatigue
In addition, the nasal spray flu vaccine may cause cold-like symptoms as the weakened virus infects the nasal cells to trigger the immune reaction. Children may experience a runny nose and wheezing. Adults may have a runny nose, sore throat, and cough.
You Are Sick—But With Something Else
Remember, too, that the flu shot only protects you from influenza—not other infections like the common cold, respiratory syncytial virus (RSV), and other viruses that may cause flu-like symptoms. Adults typically catch two to four colds per year, and young children will get six to eight.
You Have an Unrelated Case of the Flu
It takes about two weeks for the flu vaccine to start working. If you’re exposed to an influenza virus during this period, you can still get sick with the flu. Also, you may have been exposed to a flu virus shortly before getting your shot.
Lastly, the strains of flu included in the flu shot vary from year to year. Unfortunately, it is particularly prone to change, with new strains appearing often. Scientists work to target the strains that will be the most prevalent that season so that the vaccination can be tailored accordingly. Despite their best efforts, they may sometimes get it wrong.
A Word From Verywell
Everyone wants to do what they can to stay well. But avoiding the flu vaccine because you are concerned that it will make you sick is unnecessary and potentially dangerous.
The annual influenza vaccine is recommended for everyone aged 6 months and older. Flu vaccination prevented roughly 7.5 million illnesses and 6,300 deaths due to flu during the 2019-2020 season. Still, around 38 million Americans became sick with the flu, leading to 22,000 deaths. That number could have been greatly reduced if more people got the flu vaccine.
