As with any vaccine, it is essential to closely monitor the safety and effectiveness of COVID-19 vaccines as they are delivered. If a problem is reported following vaccination, health authorities will perform a thorough investigation to assess if the reported side effect is causally related to the vaccination.

During these investigations, it is extremely rare that health problems are found to be caused by the vaccine itself. Adverse events are most often found to be coincidental in time with the vaccination and may be entirely unrelated to vaccination. Sometimes they are related to how the vaccine has been stored, transported, or administered.

In the very rare cases where a genuine adverse reaction is suspected or there is an accumulation of reported side effects, the vaccine (or the specific vaccine batch) may be suspended from use. Further investigations will take place to determine what exactly caused the event, and corrective measures will be put in place. WHO works with vaccine manufacturers, health officials and other partners to continuously monitor any safety concerns and potential side effects on an ongoing basis.