Based on international references, somewhere between 1% and 10% are reported in Canada. We don't know exactly, because there is no way to exactly estimate the real occurrences.
One concept that is important to understand is that a spontaneous ADR system is not necessarily there to collect all drug reactions; it's there to identify what we call early signals--that is, unpredictable or unknown occurrences of events with drugs that have been recently marketed. Usually people will identify them, especially if they are close to the time the drug was administered and especially if they are serious. Either they are life-threatening or they bring people to a hospital, so these will tend to be reported.
A spontaneous ADR system generates signals that help us identify areas that we need to further investigate. It's not a way to monitor drugs, but a way to identify safety issues that need to be further investigated. Once we identify it and an adverse drug reaction reports an area of concern, then we seek information from other sources, such as medical literature and other regulatory agencies. The manufacturer potentially might have completed some studies that would help us assess the issue. Then we do what we call a single assessment, which is basically a more comprehensive evaluation of the safety issue.