How to understand data on side effects

UMC receives increasing amounts of questions from the public and the media regarding reports of suspected side effects to Covid-19 vaccines. Here we have outlined some questions and considerations to guide the public on these topics. For more information, please contact your national regulatory authority.

I want to know how many side effects have been reported for COVID-19 vaccines, can you help?

Unfortunately, it’s not that simple. The database we maintain on behalf of WHO, called VigiBase, contains reports from all over the world of cases where a side effect is suspected of being related to a certain medicine or vaccine. But it still takes a lot more work to analyse those reports and investigate those suspicions. Often, the analysis does not find a link between the reported side effect and the medicine or vaccine. For this reason, the preliminary data in VigiBase must be interpreted with great care. We explain more in the sections below.

What is VigiBase?

VigiBase is a global database containing reports of potential side effects of medicinal products, also known as adverse drug reactions. The report data in VigiBase comes from the member countries of the World Health Organization Programme for International Drug Monitoring (WHO PIDM), which send their reports to Uppsala Monitoring Centre (UMC). UMC develops and maintains the database, and supports the science of medicine safety. UMC is an independent, non-profit foundation and has no regulatory role or authority.

How is the VigiBase data used?

VigiBase is designed to be used by health professionals with expertise in evaluating medicine safety. Gathering national data into a global database helps researchers detect safety issues that might not be apparent from the data of a single country. This increases the probability of detecting rare adverse drug reactions.

But there are a few considerations to keep in mind when interpreting VigiBase data:

  • Although the database covers most of the world’s population, some countries are more represented than others because they send in more reports.
  • The medicines safety systems vary from country to country. In some countries, the practice is to report all adverse events. In others, the practice is to report only those suspected to be caused by a medical product, following an assessment. And each country chooses what to submit to the database and when to submit it – so the reports in VigiBase may not be a complete representation of all the reports that emerge in countries.
  • Only a small percentage of reports in VigiBase are related to vaccines. Most reports relate to suspected adverse reactions to medicines.

Does a report in the database mean there is a confirmed side effect?

No. There is a big difference between a suspected side effect and a confirmed side effect. The reports in VigiBase are of suspected side effects (and as noted above, the level of suspicion required for a report can vary from country to country). In general, think of these reports as flags that someone has put up to say “hey, this might be something; we should look into this further”. But it’s not possible to confirm whether a medicine or vaccine is causing the reported side effect unless there is enough data to analyse. Even then, a thorough scientific assessment has to be carried out, which requires time and expert resources. In other words, a simple search of VigiBase will not reveal the safety profile of any vaccine or medicine.

But I just want a list of suspected side effects!

A list of suspected side effects does not paint an accurate picture of the situation. It won’t tell you important information about the severity or duration of the problem or how soon it occurred after the medicine was taken. A scientific assessment is needed to interpret all the data in context to confirm that the observed side effect is likely to be caused by the medical product. Without that assessment, the list can be extremely misleading.

Who can view the VigiBase data?

Members of the WHO PIDM can access all the data in VigiBase.

People with a degree in one of the health professions may also be granted access for a fee upon request, as described here.

The WHO also offers a web tool called VigiAccess, which allows health professionals and researchers to search publicly available data without having to request access. However, the public view of the data is aggregated at the WHO region level. Country level data and individual reports are not available in this view. Also, all results for the various COVID-19 vaccines are currently aggregated into a single result, and it's at present not possible to see which suspected side effects are reported for which vaccine.

Why won’t UMC make more data public?

UMC is the custodian of the data held in VigiBase, but not the owner of it. In making some parts of the VigiBase data available, UMC is bound by different data protection laws from around the world and agreements between member countries and the WHO. Every record in VigiBase represents a patient and we are legally bound to protect their private information.

Also, it would not be responsible to release preliminary data out of context and without a sound scientific explanation. The data we maintain, and the analyses of that data, are shared with our primary stakeholders – WHO and WHO PIDM members.

Where should I turn for information?

If you would like to request more information, please contact the medicines regulatory authority in your country.

Last modified on: February 22, 2024