Although the National Childhood Vaccine Injury Act of 1986 requires doctors and other health care providers who administer vaccines to make a report to VAERS for “hospitalizations, injuries, deaths and serious health problems” following vaccination, it is estimated that this happens in only between 1-10% of the cases.8 9 VAERS receives about 30,000 reports annually.

http://www.thevaccinereaction.org/2015/11/gauging-vaccine-reactions-absent-reliable-reporting/