Revised: 3 July 2013
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 keeps the safety of all vaccines approved for use in New Zealand under continual review 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 and 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 then 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:
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.
Aadverse 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. Moreover, summaries of reported events should not be used to try and directly compare the relative safety of vaccines.
With any vaccine the following types of suspected adverse reactions are expected to be reported:
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.
It is important to note that 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 background 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 immunisation 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 with the background rates it is important to:
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 million, is approximately 2800 cases per year.
If 100,000 people were vaccinated, we would expect the following number of seizure reports:
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.