As useful a tool as VAERS is, it has several important limitations. One of these that is, perhaps, most important when dealing with anti-vax claims is that it does not establish causality. In other words, the reports found in VAERS cannot be taken at face value as meaning that a vaccine caused a specific effect. Many of the reports may be merely coincidental. Some may be spurious. Others may actually have a real, rather than merely apparent, connection between the vaccine and the outcome.
An oft-quoted example of just how unreliable VAERS can be can be found from James R. Laidler, MD. Dr. Laidler submitted a report to VAERS that the influenza vaccine had turned him into The Hulk. The report was accepted and posted to the database. As Dr. Laidler reports, he was contacted by a representative because the AE was so unusual. They asked his permission to delete the report from the database, after discussing it with him, and he agreed. Had he not given his permission, the report would still be in the VAERS database.
In the end, while VAERS can be a good tool for finding possible trends of vaccine-related adverse events, it is also open to being skewed by bad data. Compound this with lobbying, grass-roots efforts to get people to report certain events, and the data become horribly polluted with potentially misleading information. Why is this important? Well, it means that when searching VAERS, the results need to be taken with a substantial grain of salt, and that further, more in-depth inquiry is needed. Secondly, it means that mass-reporting of spurious connections can divert scarce resources away from investigating real health risks to chase after ghosts.
huh???