Understanding Post-Vaccination Illness: Flu Shot and COVID Booster Side Effects Explained

The reason some people feel sick after receiving the flu shot or a COVID booster is likely due to the immune response triggered by the vaccine. Vaccines simulate an infection, causing the immune system to respond and produce antibodies to fight off the simulated infection. Feeling unwell after vaccination is a sign that the immune system is working to prevent severe illness. Common side effects include soreness, redness, swelling at the injection site, as well as flu-like symptoms such as headache, fever, muscle aches, fatigue, and nausea. These symptoms typically last for a few days. Lack of symptoms does not mean the vaccine is not working, as research has shown that individuals with more symptoms tend to have a stronger antibody response. It is important to seek medical attention if symptoms persist for longer than a day or if there are signs of a true infection.