Under recommended conditions, all vaccines used in national immunization programmes are safe and effective if used correctly. In practice, however, no vaccine is completely risk-free and adverse events can occasionally result after an immunization.

Adverse events can range from minor side-effects to more severe reactions. They can be a cause of public concerns about vaccine safety. To understand a specific event and to be able to respond appropriately, there are several questions that you need to answer:

  • What caused the reaction?
  • Was it related to the vaccine or the way it was administered, or was it unrelated?
  • Are the reactions minor or severe?

This module will help you to answer these questions. You will look at the main types of adverse events and the situations in which they may occur. You will also be introduced to the challenges and opportunities of mass vaccination campaigns. Because of the nature of these campaigns, adverse events may be more noticeable.

Module outcomes

By the end of this module you should be able to:

  • 1Define the main types of adverse events following immunization (AEFIs),
  • 2Differentiate between a reaction related to the vaccine itself, to the vaccination procedure (immunization error), or to coincidental events that are not linked to the vaccine,
  • 3Differentiate between minor and severe vaccine reactions,
  • 4Describe potential underlying causes for each type of AEFI, and understand the link between the AEFI and its cause,
  • 5Summarize the expected incidence of the different types of AEFI.