Vaccine Adverse Event Reporting System reporting source: A possible source of bias in longitudinal studies

OBJECTIVE. The US Vaccine Adverse Event Reporting System (VAERS) is a passive reporting system to which anyone can report an event. Publicity related to potential adverse events may change reporting patterns. The objective of this paper is to show how litigation-related reports have influenced the trends in possible adverse event reports to VAERS. METHODS. The VAERS public-use data files were downloaded in July 2004 and translated into identical SAS data sets for analysis. Cases that were related to litigation were identified using a word search algorithm. All cases for the most frequently reported symptoms in litigation (overdose, neuropathy, autism, "mental retardation," arthralgia, and "speech disorder") were reviewed. RESULTS. In recent years, most case reports to VAERS that were related to overdose, neuropathy, and thimerosal were related to litigation. Many cases that were related to autism and mental retardation were as well. CONCLUSIONS. This review shows a previously undisclosed rise in the number of reports to the VAERS related to pending litigation for vaccine injury. The implications of this for understanding longitudinal reporting patterns are discussed.