What reactions are likely after an immunization?
Most reactions to vaccines are mild and self-limited. These are usually limited pain, swelling, and redness at the site of the vaccination. These occur in up to 80% of individuals and start within hours of the vaccination. Some people can get more generalized symptoms, including fever, muscle aches, headache, loss of appetite, and feeling generally tired. These systemic (generalized) reactions are seen more commonly with live attenuated vaccines and usually occur seven to 21 days after the vaccine was given. The worst (and very uncommon) reaction is anaphylaxis (a severe allergic reaction). These reactions usually occur shortly after the vaccine is given and can be life-threatening. Fortunately, these reactions only occur two times for every million doses of vaccine given.
