A very large number of adverse events, including those immediately following vaccination, go unreported. All reports are done on a completely voluntary basis. There is no penalty of law for not reporting.
Therefore, all deep studies using VAERS (Vaccine Adverse Event Reporting System) established in 1990 will not be telling the true story in the U.S. Further, the CDC and FDA jointly established VAERS, so these are the numbers they use in their reporting, and they do not remind nor educate their readers that the adverse event statistics are in reality many times higher due to under reporting.
"Underreporting" is one of the main limitations of passive surveillance systems, including VAERS. The term, underreporting refers to the fact that VAERS receives reports for only a small fraction of actual adverse events." - VAERS, 2014
Serious adverse reaction - "A total of 14.2% of all reports described serious adverse events, which by regulatory definition include death, life threatening illness, hospitalization or prolongation of hospitalization, or permanent disability." – VAERS, 2003
The reporting is only done for a period between 2 -14 days after injection, CNN, 2007. But many reactions occur much after 14 days. The HPV vaccines have been documented to cause neurological and disabling effects weeks after inoculation. As well as DTaP and MMR have been shown to cause inflammation in the brain causing encephalitis and autism. These diagnosis may show up immediately following vaccination, but for many, take months to see.
All the while, America's governmental agencies have actively and quietly surpressed any negative information on vaccine safety and efficacy from becoming prominent issues.
In 1984, the Food and Drug Administration (FDA) under the umbrella of the Department of Health and Human Services, internally mandated a restriction the public discussion of negative reports conducted on the Oral Polio vaccine adminstered nation wide since the 1950's.
"However, although the continued availability of the vaccine may not be in immediate jeopardy, any possible doubts, whether or not well founded, about the safety of the vaccine cannot be allowed to exist in view of the need to assure that the vaccine will continue to be used to the maximum extent consistent with the nation's public health objectives." - FDA - Federal Register Vol. 49, No. 107, 1984