Revised: 12 June 2019

Safety Information

How Does Medsafe Monitor Vaccine Safety?

How does Medsafe monitor monitor vaccine safety?
What events do we expect to be reported after immunisation?
How can real adverse reactions be distinguished from coincidental medical events?

How does Medsafe monitor vaccine safety?

All vaccines have the potential to cause adverse reactions in some people. Most adverse reactions following vaccination are mild and short-lived. However, some vaccines may rarely cause more serious reactions.

Medsafe continually reviews all vaccines approved for use in New Zealand and takes appropriate action if any risks are identified.

Medsafe routinely uses information from many different sources to monitor vaccine safety including:        

  • clinical and epidemiological studies        
  • case reports      
  • published literature        
  • pharmaceutical companies       
  • other regulatory authorities.

In addition, vaccines adverse events are reported to the Centre for Adverse Reactions Monitoring (CARM). CARM inform Medsafe and the Medicines Adverse Reactions Committee if they detect a possible safety problem with any vaccine. These reports are often referred to as adverse events following immunisation (AEFI) rather than suspected adverse reactions.

Healthcare professionals and consumers are encouraged to report any suspicions that an event may have been caused by vaccination. Reports sent to CARM may describe:

  • real adverse reactions to the vaccine
  • anxiety events or nervousness about needles or the process of vaccination
  • coincidental events that would have occurred anyway.

For these reasons, summaries of reported events following immunisation are not lists of known or proven adverse reactions to vaccines and must not be interpreted and used as such.

Adverse event reporting rates are very variable and dependent on many factors. Therefore, these data cannot be used to determine the frequency of adverse reactions to vaccines in the whole population. Summaries of reported events should not be used to try and directly compare the relative safety of vaccines.

How can I report an adverse reaction to a medicine?

What events do we expect to be reported after immunisation?

With any vaccine the following types of suspected adverse reactions are expected to be reported.

  • Injection site reactions (eg, pain, swelling).
  • 'Well-recognised' events such as headaches, dizziness, muscle aches, mild fever and tiredness.
  • Mild allergic reactions such as mild rashes and itching.
  • A few serious allergic reactions (healthcare professionals giving vaccines are trained to spot the symptoms of serious allergic reactions and treat them).
  • Events due to fear or anticipation of the needle injection, such as fainting.
  • Coincidental medical conditions (eg, neurological disorders).
  • New adverse reactions (ie, those not already listed in the data sheet).

In New Zealand it is unlikely that rare side effects to vaccines will be detected as the population immunised is usually too small. Therefore, Medsafe uses international data to help determine if these rare events may be due to immunisation.

How can real adverse reactions be distinguished from coincidental medical events?

As vaccines are given to large sections of the population or preferentially to those with underlying medical conditions (as is the case for the seasonal 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. Events that occur more than 60 days after vaccination are unlikely to be due to the vaccine.

Another tool 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 conclude that a definite diagnosis of the event reported was made
  • adjust the expected rate for differences in different populations and seasonal variations.

Example: How could we compare the observed versus expected rates for seizures (fits)?

Published data estimates the incidence of seizures in those over 18 years in European countries to be around 70 cases per 100,000 people per year (in children the incidence is higher).

The background incidence for the whole of New Zealand, population around 4.8 million, is approximately 3,306 cases per year.

If 100,000 people were vaccinated, we would expect the following number of seizure reports:

  • 0.17 reports within 1 day of vaccination
  • 1.2 reports within 1 week of vaccination
  • 7 reports within 6 weeks of vaccination1

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.

Where can I find information about vaccines?

What safety communications has Medsafe issued?

How does Medsafe monitor medicine safety?

How can I find information on reports of suspected adverse reactions to vaccines reported to CARM?


  1. Black S, Eskola J, Siegrist CA et al. 2009. Importance of background rates of disease in assessment of vaccine safety during mass immunisation with pandemic H1N1 influenza vaccines. Lancet 374: 2115-22.
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