The COVID-19 vaccine booster shot's side effects appear to largely mirror how people felt after their second dose, according to a study published Tuesday by the CDC. The report found that the side effects were mostly considered mild or moderate, and arm pain, fatigue and headache were the most commonly reported symptoms after the third shot. The side effects kicked in generally the day after the injection, the report found, and 28% of people said they were unable to perform normal daily activities because of them.