This is an important limitation of VAERS. It cannot be used, on its own, to figure out if an event following vaccination is common or rare. For example, when vaccines against the SARS-CoV-2 coronavirus started to be used in the population at large, more and more reports of adverse events began to be submitted to VAERS. This should not come as a shock. Obviously, the more a vaccine is used, the more adverse events are likely to be reported. If a vaccine requires two doses instead of one, that’s twice as many opportunities for an undesired event to take place after a shot.
Moreover, if the background rate of the event being reported, meaning how common this event is in the population, goes up, it’s likely that its frequency in VAERS will also go up. If the diagnostic criteria for a condition are expanded, more people will receive the diagnosis, which will make it look like its incidence has suddenly increased. This is what happened with autism over the years, and since the diagnosis for autism is usually made around the time a child receives a number of vaccines, it’s easy to witness more reports of autism being filed following vaccination and to wrongly claim causation. VAERS can also be biased in an interesting way: mild events following a vaccine are less likely to get reported compared to more severe ones. Finally, there is also the fact that as an adverse event gets widely reported in the media, more people will pay attention to it, leading to an increase in reports to VAERS. This is known as stimulated reporting.