Understanding Side Effects and Adverse Events

Adverse Event

An “adverse event” is any health problem that happens after a shot or other vaccine. An adverse event might be truly caused by a vaccine, or it might be pure coincidence.

Types of adverse events include:
  • True reactions to the vaccine.
    These include both common, known side effects and serious reactions, like allergic reactions.

    Side Effect
    side effect is any health problem shown by studies to be caused by a vaccine. Like any medication, vaccines can cause side effects. Usually vaccine side effects are minor (for example, a sore arm where a shot was given or a low-grade fever after a vaccine) and go away on their own within a few days.

  • Unrelated health problems.
    These are experiences that would have occurred even if the person had not been vaccinated. They happen after vaccination but are not caused by the vaccine.
  • Health problems that cannot be related directly to the vaccine.
    The cause of these events is unknown, and there is not enough evidence to say whether they are caused by a vaccine.

One of the main jobs of CDC’s Immunization Safety Office is doing research to find out if adverse events that are reported by doctors, vaccine manufacturers, and the public are truly caused by a vaccine.

CDC Vaccine Information Statements and the Safety Information by Vaccine webpage provide information about common and rare side effects of recommended vaccines.