Reporting vaccine adverse events

Importance of vaccine safety monitoring

Like all drugs, no vaccine is 100% safe or effective. As vaccines are generally given to healthy people to prevent diseases, the tolerance (by the public, policy-makers and healthcare professionals) to vaccine adverse events (AEs) is substantially lower than that for drugs for disease management. It is important that we have a robust vaccine safety monitoring system that can effectively detect early safety signals of concerns and mitigate them. This will help to ensure that the vaccines continue to be safe and effective for use. Spontaneous reporting of AEs is an important post market safety surveillance tool for HSA to promptly detect vaccine safety signals.

a) Vaccine AE classification by World Health Organisation (WHO)1

The WHO classifies AEs following vaccinations into:

  1. Vaccine product-related reaction – event caused or precipitated by a vaccine due to one or more of the inherent properties of the vaccine product. It is worthwhile to note that vaccine AEs can be reactions expected for all vaccines such as injection site reactions or specific events known to be associated with a vaccine type e.g. 1976 swine influenza vaccine and Guillain-Barré syndrome.
  2. Vaccine quality defect-related reaction – event caused or precipitated by a vaccine due to one or more quality defects of the vaccine product, including the administration device provided by the manufacturer
  3. Immunization error-related reaction – event that is caused by inappropriate vaccine handling, prescribing or administration and that thus, by its nature, is preventable
  4. Immunization anxiety-related reaction/Immunization stress-related response – event arising from anxiety about the immunization
  5. Coincidental event - event that is caused by something other than the vaccine product, immunization error or immunization anxiety

b) Causality assessment of individual AE reports by HSA

For causality assessment in the clinical setting, healthcare professionals can take a pragmatic approach by assuming that any AE which occurs within a reasonable timeframe after vaccination may be caused by the vaccine, regardless of whether it is truly causal or coincidental.

Upon receipt of the AE report, HSA will conduct an in-depth assessment, taking into account the following:

  1. previous local and international experience with the vaccine (i.e. whether similar events had been observed among other vaccinees or non-vaccinees based on international signals and published literature, safety data from animal studies where available);
  2. alternative causes;
  3. biological plausibility that vaccine could cause the event
  4. timing of the event;
  5. characteristics of the event (e.g. laboratory findings); and
  6. rechallenge (if undertaken).


Details required for reporting of AEs following vaccination

As vaccines are biologically-derived products, additional information useful to aid in causality assessment of vaccine AEs include:

  1. brand name of the vaccine,
  2. lot or batch number,
  3. route of administration
  4. number of doses of vaccine given
  5. concurrent vaccinations

It is recommended that as much details are provided on the AE as possible to enable HSA to make a causality assessment.  



Healthcare professionals are encouraged to report serious AEs suspected to be associated with vaccines to HSA. Healthcare professionals can report AEs to HSA via this link:'-guide-to-adverse-events-reporting, or through the CMIS module if available in their hospital electronic system. Healthcare professionals may also refer to the above website for HSA's video guide on AE reporting.

A robust vaccine monitoring system in Singapore to safeguard public health can be achieved with the active participation of healthcare professionals.



  1. Causality assessment of an adverse event following immunization (AEFI). Updated user manual for the revised WHO classification. 2019

Healthcare professional, Industry member, Therapeutic Products

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