Revised: 21 November 2023

Safety Information

How Does Medsafe Monitor Vaccine Safety?

Distinguishing real adverse reactions from coincidental medical events
Adverse events of special interest
More information

The general principles for monitoring medicines also apply to vaccines. Find out about how Medsafe monitors medicine safety.

However, there are some additional considerations for monitoring vaccines, which are described below.

Distinguishing real adverse reactions from coincidental medical events

As vaccines are given to large sections of the population or preferentially to those with underlying medical conditions (eg, the flu vaccine) there will a number of coincidental events that may be reported to be due to the vaccine.

The challenge is to be able to distinguish coincidental events from those that may have been caused by the vaccine.

The time between vaccination and an event can be important in determining whether the event was coincidental. Most reactions to vaccines occur within a very short time of vaccination, usually within days.

Another tool for monitoring vaccine safety is to compare the number of reports for an event with the expected background rate. When comparing the reported rates of events (also called the observed rate) with the background rates it is important to:

  • determine whether the reports include enough information to diagnose the event
  • adjust the expected rate for differences in different populations and seasonal variations.

A greater rate of observed versus expected reactions may indicate that a safety signal exists for that vaccine. However, Medsafe needs to consider the limitations of under-reporting and the quality of the reports when determining if there is a safety signal.

Adverse events of special interest (AESI)

Adverse events of special interest (AESI) are pre-specified medically significant events that have the potential to be causally associated with the vaccine and must be carefully monitored. AESI can be serious or non-serious and can include:

  • events of interest due to their association with the disease being vaccinated against
  • events of interest for vaccines in general (eg, to the specific vaccine type or adjuvants).

The AESI list changes based on the evolving safety profile of a vaccine. Although these adverse events may occur after being vaccinated with a vaccine, they may not necessarily be related to the vaccine.

More information

For more information about the known adverse reactions for a vaccine, see the consumer medicine information (CMI) or data sheet, or ask your healthcare professional.

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