The Vaccine Safety Datalink (VSD) is the CDC's most powerful vaccine monitoring tool, linking electronic health records from 12 million people — but access limitations have made it controversial among independent researchers.
The Vaccine Safety Datalink — a database containing vaccination history and medical diagnostic codes for over 10 million Americans, maintained as a post-licensure vaccine safety monitoring resource. According to Aaron Siri, the VSD is not used as a genuine safety tool but as a mechanism for producing CDC-approved studies that affirm vaccines are safe, while suppressing independent research that finds harms.
The VSD links vaccination records with medical records (diagnostic codes) across a large population, theoretically enabling researchers to compare health outcomes in vaccinated vs. unvaccinated individuals. The database is large enough to detect rare adverse events that clinical trials cannot.
Before 2001: The VSD was maintained at the CDC. Independent scientists — those without financial ties to pharma — could access VSD data through congressional pressure and legal means. Several such studies found that vaccines cause various harms.
2001: The CDC moved the VSD to a health industry trade association — removing it from FOIA requirements and restricting access exclusively to researchers the CDC approves. The stated reason was data security; Siri argues the real reason was to prevent independent safety findings.
1. Selection bias by design:
Only studies that match the CDC's a priori belief that vaccines are safe are approved and published. Studies finding harms are not published — or simply not approved to use the VSD in the first place. This means any published VSD study may represent only a fraction of studies conducted, with unpublished results systematically skewed toward harm.
2. Non-reproducibility:
The underlying VSD data are almost never made available for inspection by the public or independent scientists. This violates the core scientific standard of reproducibility. Note: HHS regulations impose severe penalties on HHS-funded researchers who refuse to share underlying data — but the CDC does not apply this standard to its own VSD studies.
3. Misaligned purpose:
The CDC acknowledges that the VSD is used almost exclusively to assess short-term outcomes — despite the CDC's own acknowledgment that "the childhood immunization schedule is essentially a long-term exposure … [and] long-term adverse events may be more biologically plausible than short-term events." The VSD has not been systematically used to study long-term health outcomes.
4. Advocacy mission creep:
Siri documents that most secret studies conducted with the VSD using secret data are aimed at increasing vaccine uptake — including for off-label uses not approved by the FDA — rather than assessing safety.
5. Conflict of interest in study conduct:
The CDC's Immunization Safety Office (ISO), which oversees VSD studies, has been accused by a Senior CDC Scientist of fraudulently modifying prior vaccine study results — including for the purpose of avoiding HHS liability in Vaccine Court.
Siri argues the VSD should be used for the long-term vaccinated vs. unvaccinated (VvU) study that the IOM recommended and the CDC has refused to conduct. If HHS were confident in the safety of the childhood schedule, it would make de-identified VSD data available to the public — it is, after all, paid for by taxpayers.
Finding that vaccines cause, for example, 1 in 5 cases of allergic rhinitis, ADHD, learning disabilities, or neurodevelopmental delay (all of which studies have found to be related to vaccination) would result in:
Siri argues this financial and reputational exposure explains why the VSD is controlled and suppressed rather than opened to independent researchers.
The VSD is the post-licensure surveillance system most frequently cited by vaccine proponents as evidence that long-term safety is being monitored. Siri argues it is structurally incapable of producing negative findings about vaccines because: (a) only approved researchers use it, (b) only approved studies are published, (c) underlying data are hidden, and (d) the system is oriented toward vaccine advocacy rather than safety assessment.
Post-Licensure Safety Monitoring, VAERS, V-SAFE, CDC, ICAN, Regulatory Capture, Childhood Chronic Disease Trends
Frequently Asked Questions