VAERS is the Vaccine Adverse Event Reporting System put in place in 1990. It is a voluntary reporting system that has been estimated to account for only 1% (see the Lazarus Report) of vaccine injuries. OpenVAERS is built from the HHS data available for download at vaers.hhs.gov.
Apparently a team of developers have build this web app around the VAERS data and put in some powerful searches. VAERS itself was developed 34 years ago, and deployed in 1990! We've already caught the CDC purging records, and this team has likely have retained the original ones. (FRN)